A Sweeper (also know as a Libero) is a defender, usually a CB, who sweeps up the ball if an opponent breaks into the defensive line. A sweeper is more active and keep their positions more fluid and dynamic rather than just man-marking their designated opponents. Usually, sweepers are more creative and have a good mentality attribute.
Sweepers may also be expected to build counter-attacks, and that is why a sweeper is good at ball control, passing (especially long balls) and they have good vision to trigger a counter-attack.
Sweeper in FIFA
In FIFA, there is no such a specific role or position as Sweeper, but CBs with good ball control, passing, vision and pace attributes can play the role of a sweeper.