Definition of Hi-Lo
By default, Hold'em is a Hi game, meaning that the goal at the showdown is to get the highest 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 split in two, one part for the high hand and one part for the low hand (Hi-Lo for high-low). Omaha Hi-Lo is also called Omaha-8. It is called Omaha-8 because the 8 is the highest of the low cards. A 9 is a Hi while an 8 and lower is a low. The ace in these variants can be a Lo and a Hi.
Details on Hi-Lo variants
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 hole cards. In this situation, the player with the Hi will win 100% of the prize pool. 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.