It’s a list of files that are not in .gitignore (which can be versioned), but which you want to exclude on your machine’s copy of the repo.
From the docs:
Patterns which are specific to a particular repository but which do
not need to be shared with other related repositories (e.g., auxiliary
files that live inside the repository but are specific to one user’s
workflow) should go into the $GIT_DIR/info/exclude file.
Note that .git/info/exclude can not be versioned because it’s in the .git directory.