it means when a runner is on base and the batter is retired by flying out, then runner can then go on but only if the runner leaves the base AFTER the ball is caught. So if there is a pop fly/ fly ball and it looks like it can be catches then the runner return to their previous base and waits. Then when the ball gets caught they can decide if they want to keep going or not. So long story short returning to the base is used as the term tagging up.
