A video game trope, this involves instances where you must go back through a section (whether a single room, an entire level, or more) you have already visited before.
When done badly, backtracking can be one of the purest types of filler there is, with you being forced to go through lots of already-explored areas for no reason other than to extend the play-time of the game.
Due to this, backtracking tends to be much-maligned. However, backtracking in itself is a neutral trope that can be used for good or evil, and certain genres even have it as a central gameplay aspect.
When done right, backtracking can make an area feel far more real than if you just followed a straight line through it. It can also show contrasts, such as a familiar area going through major changes (such as exploring the Doomed Hometown before and after its destruction), or having familiar scenery but completely different enemies and gameplay. Also, in games where the protagonists get more abilities over time, heading back through previous areas much faster due to newly accessible shortcuts (as well as new areas) and blowing everything away easily can contrast with how much trickier navigating and surviving a location was before (this can be one of the biggest strengths of the Metroid Vania genre).
Of course, sometimes these can merge or cross-over, so, for instance, backtracking through a familiar area under total safety (the kind of sequence that generally comes under Filler) could help you wind down after a particularly intense section, or something seemingly from the "good" type can be handled poorly and feel more like Filler (something Metroid Vania games do occasionally).
Compare Level in Reverse, where the route you take through the level and/or the level geometry itself is reversed/rotated, and everything else is changed.