PLO stands for "Pot Limit Omaha". 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 in Omaha, the preflop equity between hands is closer together. It makes sense to reduce variance by taking more flops and therefore limiting preflop raises. In Hold'em, it is possible to have preflop spreads between hands of 80%-20%. In Omaha, when your hand is very strong, you will rarely have more than 65% equity.
Some PLO basics
Pot Limit Omaha is by default a "hi" game, meaning you have to make the highest hand. There are other types of Omaha such as Omaha Hi-Lo where you have to make both the highest and the lowest hand. Pot Limit Omaha is very similar to Hold'em. One of the biggest differences is that the Omaha player will be dealt 4 cards instead of 2. To make his hand, he will have to choose two cards from his hands and 3 cards from the community cards.