Omaha Hi-Lo is a "split pot" game, meaning that at showdown, the pot is divided in half, with one half being awarded to the winning best hand, and the other half to the best qualifying "low hand."
There are no qualification requirements for the high, meaning there will always be a winning high hand. However, low hands must qualify to be eligible for winning the low half of the pot. If there is no low hand, the high hand will scoop the entire pot.
A more comprehensive explanation can be found here.