6-max is a popular online poker format in which there is a maximum of six players at the table. This is in contrast to the standard full-ring poker format that features 9-handed games.
This short-handed poker format is the prevalent structure in online poker cash games, as it enables players to see more action. You will also find some MTTs with six-handed tables, but these are more of an exception than a rule.
Moreover, 6-max games are generally looser, have more hands per hour, and see bigger pot sizes. The smaller field also means that the players should adjust their playing style.
For example, in a 6-max poker game, you should adjust your opening ranges to widen it a bit since you have fewer players to go through.