PLO means "Pot Limit Omaha". The Pot Limit Omaha is the most popular variant of Omaha. If the most popular Hold’em is "No Limit", the most popular Omaha is "Pot Limit". The reason being that on Omaha, the equity preflop in the hands are closer together. It is therefore wise, to reduce the variance, to take more than flop and thus, to limit the number of raises preflop. At Hold’em, it is possible to have variances between hands of 80%-20%. At Omaha, when your hand is very strong, you will rarely have more than 65% equity.
Some PLO basics
The Pot Limit Omaha is by default, a game of "hi", i.e. you have to make the highest hand. There are other types of Omaha like the Omaha Hi-Lo where you have to make both the highest and the lowest hand. The Pot Limit Omaha is very similar to the Hold’em. One of the biggest differences is that the Omaha player will be dealt 4 cards instead of 2, and to make his hand, he must choose two cards from his hands and 3 cards from the community cards.