“In the sport of association football, a defender is an outfield player whose primary roles are to stop attacks during the game and prevent the opposing team from scoring goals. Centre backs are usually in pairs, with two full-backs to their left and right, but can come in threes with no full backs. There are four types of defenders: centre-back, sweeper, full-back, and wing-back. The centre-back and full-back positions are essential in most modern formations. The sweeper and wing-back roles are more specialized for certain formations depending on the managers choice of play and adaptation. … The sweeper (or libero) is a more versatile centre-back who ‘sweeps up’ the ball if an opponent manages to breach the defensive line. This position is rather more fluid than that of other defenders who man-mark their designated opponents. Because of this, it is sometimes referred to as libero, which is Italian for ‘free’. …”
Guardian – The Question: Could the sweeper be on his way back? (Jonathan Wilson – Sep. 2009)
Football Tactics for Beginners- The Sweeper Keeper: From Lev Yashin to Manuel Neuer (Dec. 2017)
YouTube: What Happened to Sweepers in Football?, What is a Libero?, What is a Sweeper Keeper?