For all unstaged files in current working directory use:
git checkout -- .
For a specific file use:
git checkout -- path/to/file/to/revert
-- here to remove ambiguity (this is known as argument disambiguation).
For Git 2.23 onwards, one may want to use the more specific
git restore .
git restore path/to/file/to/revert
that together with
git switch replaces the overloaded
git checkout (see here), and thus removes the argument disambiguation.