Meaning of Git checkout double dashes

Suppose I have a file named path/to/file.txt in my Git repository, and I want to revert changes on it.

git checkout path/to/file.txt

Now suppose that the file is named master...

git checkout master

Whoops! That changed branches instead. The -- separates the tree you want to check out from the files you want to check out.

git checkout -- master

It also helps us if some freako added a file named -f to our repository:

git checkout -f      # wrong
git checkout -- -f   # right

This is documented in git-checkout: Argument Disambiguation.

The double dash "--" means "end of command line flags" i.e. it tells the preceding command not to try to parse what comes after command line options.