Koop mode means cooperation mode and is a multiplayer mode in computer and console games. You either play together in a team and solve missions or try to win against the other team or bots. Or you play against your fellow players, in some cases bots can also be in the game and support you. The co-op mode is part of the multiplayer modes, which include all the modes you can play with more than one person. Usually, a multiplayer mode is called a co-op mode when you have to work together to reach or win the goal.
Known examples here are the Nintendo party games in which you can compete against each other but also play together in a team against each other or against the computer. Another well-known example is the Rainbow Six tactical shooter series, which has taken Coop gameplay in the shooter area to a new level. Here it is not only a matter of good teamwork, but also timing and exact agreements about who does what are absolutely necessary to win. New on the video game market, the Koop mode has become indispensable and enjoys greater popularity than ever before.