Definition of Hi-Lo
By default, Hold'em is a game of Hi, which means that at the slaughter, the goal is to get the strongest combination. In some games such as 2-7 triple draw, the lowest hand will win the prize pool. In other games, such as Omaha-Hi-Lo, the prize pool will be divided in two, with one game for the stronger hand and one game for the weaker hand (Hi-Lo for high/low). The Omaha Hi-Lo is also called Omaha-8. It is called Omaha-8 because 8 is the highest of the small cards. A 9 is a Hi while an 8 and under is a low. The ace in these variations can be a Lo and a Hi.
Details about the Hi-Lo variations
If we take the case of Omaha Hi-Lo, there is not always a Lo, but there is always a Hi. For example, if the board is J-J-A-K-Q, we know that there will be no Lo because we will need 5 cards smaller than an 8 including the hidden cards. In this situation, the player with the Hi will win 100% of the jackpot. If at the showdown two players have the same Lo and player number 1 also has the Hi, player number 1 will win 75% of the prize pool and player number 2 will win 25% of the prize pool.