The `refs` folder contains files which indicates the latest revision of the given reference. For example,
if we have previously fetched a file from the `main` branch of a repository, the `refs`
folder will contain a file named `main`, which will itself contain the commit identifier of the current head.  
If the latest commit of `main` has `aaaaaa` as identifier, then it will contain `aaaaaa`.  
If that same branch gets updated with a new commit, that has `bbbbbb` as an identifier, then
re-downloading a file from that reference will update the `refs/main` file to contain `bbbbbb`.