What does "reboot" mean when talking about latest game releases?

Solution 1:

A reboot is a way to reuse the concept behind a game/movie/etc. (backstory, gameplay mechanics, some characters/locations/plotlines) but otherwise rewrite the entire plot and characters. It's used to generate interest based on the franchise it's in, or to reuse a concept that was interesting but might be too old to make a sequel for or originally too poorly executed. It ignores and can totally change accepted canon (ie the characters and plot already established by previous ganes) if they're irrelevant to the new story (which is the difference vs. a remake) and often gives rise to a form of multiverse theory among some gamers.

A good example outside of gaming is the new Robocop movie. It's based on the same premise of a man who was seriously injured, stuck in a mostly robot body, and made to be a cop, but many of the other characters and much of the story was written from scratch.

Solution 2:

Typically, it means that all existing "canon" for the setting and characters is wiped clean.

This lets the writers create stories without worrying about contradictions happening with a large back catalogue of titles.