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

A refspec maps a branch in the local repository to a branch in a remote repository.
This makes it possible to manage remote branches using local Git commands and to configure some advanced git push and git fetch behavior.

A refspec is specified as

[+]<src>:<dst>

The <src> parameter is the source branch in the local repository, and the <dst> parameter is the destination branch in the remote repository.
The optional + sign is for forcing the remote repository to perform a non-fast-forward update.

Refspecs can be used with the git push command to give a different name to the remote branch.
For example, the following command pushes the master branch to the origin remote repo like an ordinary git push,
but it uses qa-master as the name for the branch in the origin repo.
This is useful for QA teams that need to push their own branches to a remote repo.

git push origin master:refs/heads/qa-master


You can also use refspecs for deleting remote branches.
This is a common situation for feature-branch workflows that push the feature branches to a remote repo (e.g., for backup purposes).
The remote feature branches still reside in the remote repo after they are deleted from the local repo,
so you get a build-up of dead feature branches as your project progresses.
You can delete them by pushing a refspec that has an empty <src> parameter, like so:

git push origin :some-feature

This is very convenient, since you don’t need to log in to your remote repository and manually delete the remote branch.

Note that as of Git v1.7.0 you can use the --delete flag instead of the above method.
The following will have the same effect as the above command:

git push origin --delete some-feature


By adding a few lines to the Git configuration file, you can use refspecs to alter the behavior of git fetch.
By default, git fetch fetches all of the branches in the remote repository.
The reason for this is the following section of the .git/config file:

[remote "origin"]
    url = https://git@github.com:mary/example-repo.git
    fetch = +refs/heads/*:refs/remotes/origin/*

The fetch line tells git fetch to download all of the branches from the origin repo.
But, some workflows don’t need all of them.
For example, many continuous integration workflows only care about the master branch.
To fetch only the master branch, change the fetch line to match the following:

[remote "origin"]
    url = https://git@github.com:mary/example-repo.git
    fetch = +refs/heads/master:refs/remotes/origin/master

You can also configure git push in a similar manner. For instance, if you want to always push the master branch to qa-master in the origin remote (as we did above), you would change the config file to:

[remote "origin"]
    url = https://git@github.com:mary/example-repo.git
    fetch = +refs/heads/master:refs/remotes/origin/master
    push = refs/heads/master:refs/heads/qa-master

Refspecs give you complete control over how various Git commands transfer branches between repositories.
They let you rename and delete branches from your local repository, fetch/push to branches with different names,
and configure git push and git fetch to work with only the branches that you want.