Definition of max
The expression max is used in several circumstances in poker. By max, we mean "maximum". If we say "value max" we mean that we have drawn the max maximum amount of money possible with our hand ("I pushed all in and he called, I got max-value"). If you are playing poker with limited bets preflop (for example Pot-Limit Omaha), making a raise that is called a "raise pot" preflop is a max raise. It can also be referred to as a max hand or a flop max. If you have a hand like 88 on a flop 882, you have a max to flop, the nuts in other words, no hand will beat you.
Max in games of cash games
In parts of cash games, the table types will be identified as follows: 6-max, 4-max, 8-max. The number indicates the number of players max or less who will be allowed to sit at the table. On a 6-max (shorthanded) table, there will be a max maximum of 6 players who will take part in the action. If you are a regular at these tables, rather than saying that you are a regular at shorthanded or 6-player tables, you can say "I play 6-max".