The term collusion is an umbrella term that describes different types of cheating activities that involve two or more players working together to gain an unfair advantage.
Colluding poker players will devise different plans to share information and dump chips to one another, all with the goal of winning as much money as possible at the expense of other players who are playing a fair game.
One common type of collusion involves sharing information about hole cards. Colluding players will use different signals to convey the information about their hole cards, providing their partners with the knowledge they wouldn’t have had normally.
Collusion can sometimes entail chip dumping as well as shady moves to squeeze non-colluding players out of the pot. For example, one partner will raise, and if another player calls, the other partner will 3-bet. When the action comes back to the first colluding player, they might 4-bet, forcing the third player out of the pot.
Unfortunately, there will always be cheaters and colluders in poker. You have to stay careful and vigilant to any suspect practices, and if you notice something out of the ordinary, you should immediately leave the game and report these unsavory activities.