The top five card rule is the rule that says your best five cards determine if you win, split with a tie or loose the pot.
The question asked "What is the “Top Five Cards” rule and how does it apply to splitting pots?" has become the post at Poker Stack Exchange that people who post hand reading questions are usually referred to when we close a question. We usually say something like this is a duplicate of the top five cards rule and hopefully provide a link here.
Its not that I am trying to be rude by closing your question, it is just questions about reading basic poker hands are very basic, and the object of a stack exchange site is to have experts to answer questions. Your question about hand reading is being asked by you because you do not understand how to read a hand or one of your poker friends is telling you something and you need to verify it. So lets get to a basic primer on how to read poker hands.
The best five cards, are simply the five cards that make your best hand. Pairs are better then ace high, two pairs are better then one, three of a kind beat two pairs, straights beat three of a kind, flushes beat straights, full houses beat flushes, four of a kind beat full houses, straight flushes beat four of a kind.
Hands that involve pairs, two pairs, three of a kind and four of a kind need kickers to complete the hand. Kickers are just the cards that are not really needed to make your two pair, four of a kind etc. but they may be needed when reading the hand when two or more players all have the same pair, same two pair etc. The most simple example of this I can give you is when four of a kind is on the board in holdem. All players have that four of a kind, and it is going to come down to kickers to determine the outcome of the hand. If you have an ace in your hand, you have four of a kind with an ace as the best five cards. If another player also has an ace then the pot is split. If there was four of a kind on board with an ace on board the pot would be split between all the players in the hand at the end. If your hole cards cannot make a better hand then the best five cards on the board, the four of a kind with an ace in this case, your hole cards simply do not count, as in this case neither do any body else's. However if instead of an ace on the board with the four of a kind there was a deuce then you hole cards would count. Those who had the single highest hole card would have the best hand and take the whole pot except when some else had the same rank of hole card, then it is a split pot. The best hand, is the best five cards.
Three of a kind work the same way. Often one of your hole cards is going to match a pair on the board in holdem. This is when kickers can count. Say you and another player both had an ace and on the flop there was a pair of aces. It would then go to kickers. If you had a higher card then your opponent in your hand and it was higher then the any one of the cards on board you win the pot. You have five better cards. Your opponent may also have the same kicker as you, but note this is still only four of the five cards needed to determine the hand. In this case it goes to the next card you might have, if your next highest card plays, meaning it with your other kicker is higher then two of the other kicker on board, and higher then your opponents you win, if it does not play then you and your opponent split the pot.
You have A7, your opponent has A6:
AA234 is the flop. you win, your hand is AAA47, your opponent hand is AAA46
AAJQK is the flop. You split with your opponent, both of you have the same hand, AAAQK
the best hand involving Straights, flushes and straight flushes never involve kickers. In the case of a straight it is always how high a straight is that determines if the pot is won or split. If you both have the same straight you split the pot, if one has a higher straight that one wins. If the entire straight is on the board it is a tie and the pot is split.
With flushes the highest flush is not determined by the top card, but by going down to the next lowest card. If you have the deuce of a suit and your opponent has a three, and four cards of that suit are on the board, your opponent wins, because the fifth lowest card in their flush is the three, while the fifth lowest card in your flush is the deuce. If their were five cards to that suit on the board, the pot would be split because neither of you has a flush that can beat the flush on the board. By the same token if you both had two cards to the flush, the best flush is the hand that is three on board and two in hand, with the highest in hand being the winner. Flushes are simple to figure out, if you both have a flush of the same suit it is simply the one whom has the highest flush card that wins. With the exception of when the flush is on board, you will need to have on of those suites higher then the lowest card of that suit on board to win.
It is always the best five cards that make a five card poker hand that determines the winner loser or split. Holdem is played with seven cards, and the two cards that are not part of the best five card hand count for absolutely nothing in determining if a pot is won lost or split. Those two cards if not part of a best five card hand do not count for anything, they are not part of the hand. If one of those too cards could improve your hand they would be part of the best five cards. It is the best five cards plain and simple, and always. Yes best five cards(always).
To quote an ex of mine, "What? You don't believe me?"
Look here at Wikipedia
Or here at poker listings
Or Google Poker Hand Rankings there are about 329,000 results, maybe about 329,001 after I post this.