An inverted full-back is a defender who lines up wide but, once their team has the ball, steps into central midfield rather than overlapping down the touchline. It thickens the middle of the pitch for building play and leaves players better placed to stop a counter-attack the moment possession is lost, instead of being stranded upfield out wide.
Pep Guardiola made the role well known at Bayern Munich from the 2013-14 season, moving Philipp Lahm, one of the best right-backs in the world at the time, into central midfield once his team settled into possession. Lahm ran games from there the way a specialist holding midfielder would.
Guardiola did it again at Manchester City with João Cancelo between 2020 and 2022, tucking him inside to add an extra body in midfield during City's title-winning seasons. Other coaches have since built their own versions of the same idea.
The trade-off is pace out wide. A team built this way gives up some of the width and directness of a traditional overlapping full-back in exchange for control of the centre of the pitch, on the bet that dominating possession there matters more than a straight run down the line.