Read through GiLab's branching documentation:

See also:

Default branch

When you create a new project, GitLab sets master as the default branch for your project. You can choose another branch to be your project's default under your project's Settings > General.

The default branch is the branched affected by the issue closing pattern, which means that an issue will be closed when a merge request is merged to the default branch.

The default branch is also protected against accidental deletion. Read through the documentation on protected branches to learn more.

Delete merged branches

Introduced in GitLab 8.14.

Delete merged branches

This feature allows merged branches to be deleted in bulk. Only branches that have been merged and are not protected will be deleted as part of this operation.

It's particularly useful to clean up old branches that were not deleting automatically when a merge request was merged.