https://www.atlassian.com/git/tutorials/refs-and-the-reflog/the-reflog

The reflog is Git’s safety net.
It records almost every change you make in your repository, regardless of whether you committed a snapshot or not.
You can think of it is a chronological history of everything you’ve done in your local repo.
To view the reflog, run the git reflog command.
It should output something that looks like the following:

400e4b7 HEAD@{0}: checkout: moving from master to HEAD~2
0e25143 HEAD@{1}: commit (amend): Integrate some awesome feature into `master`
00f5425 HEAD@{2}: commit (merge): Merge branch ';feature';
ad8621a HEAD@{3}: commit: Finish the feature

This can be translated as follows:

    You just checked out HEAD~2
    Before that you amended a commit message
    Before that you merged the feature branch into master
    Before that you committed a snapshot

The HEAD{<n>} syntax lets you reference commits stored in the reflog.
It works a lot like the HEAD~<n> references from the previous section,
but the <n> refers to an entry in the reflog instead of the commit history.

You can use this to revert to a state that would otherwise be lost.
For example, lets say you just scrapped a new feature with git reset.
Your reflog might look something like this:

ad8621a HEAD@{0}: reset: moving to HEAD~3
298eb9f HEAD@{1}: commit: Some other commit message
bbe9012 HEAD@{2}: commit: Continue the feature
9cb79fa HEAD@{3}: commit: Start a new feature


The three commits before the git reset are now dangling, which means that there is no way to reference them—except through the reflog.
Now, let’s say you realize that you shouldn’t have thrown away all of your work.
All you have to do is check out the HEAD@{1} commit to get back to the state of your repository before you ran git reset.

git checkout HEAD@{1}

This puts you in a detached HEAD state.
From here, you can create a new branch and continue working on your feature.