When the defending team wins the ball — by rebound, steal or block — it must take it back beyond the two-point arc before it may score. Otherwise the basket does not count.
The classic first turnover a 5-on-5 player commits in 3x3: pull down the opponent's miss, one step, lay it in — and everybody stares at you. On a half court both teams share a single rim, so a rule is needed to stop a team scoring the instant it wins the ball back. That rule is clearing: whichever team gains possession as the defence must take the ball behind the two-point arc by pass or dribble, and only then attack. The check ball follows the same logic — after every stoppage the ball is briefly touched between a defender and an attacker outside the arc before it is live again. The typical mistake is not ignorance but hesitation: think it over after the steal or rebound and you burn the decisive seconds of a possession that only lasts twelve anyway on the trip back out. Call your clear out loud and dribble out decisively.