The 2-on-1 in soccer is when there are two attacking players and only one defender to guard both of the players. This is often a result of a quick turnover and can result in a goal scoring opportunity. A 2-on-1 can happen in many sports, but for soccer specifically, it's a fast paced situation where even the slightest step out of place will turn the ball back over to the other team.
When you are in possession of the ball during 2-on-1, it's important to utilize the other person on your team. Since there is one defender, whoever is not being covered should try to get into an open position for a scoring opportunity.
When you are the sole defender caught in a 2-on-1 situation, apply pressure to the offensive players to have them make quick decisions. However, the one drawback of applying pressure is that if you focus too much on one player, the other offender may be wide open for a shot on goal. Finding a balance between attacking each player in possession while also keeping an eye on the teammate is key.