A 'Libero' is a term in volleyball. It refers to a player who is free to move from the sidelines into a back court rotation without official substitution, given that they swap with the player in that position. Liberos can easily be identified as they have no number and usually wear a different coloured shirt.
A Libero player is one highly proficient in passing (digging), and usually swaps with a front court specialist. Or those whose forte is NOT passing.