git merge <branchname1> <branchname2>

This command is used for combining the changes made in <branchname1> and <branchname2>.

It does this by creating a new commit which incorporates the changes of both branches. The changes from <branchname1> and <branchname2> are compared and if there are non-conflicting changes in the lines of the same files, the lines are merged together automatically.

In case of conflicts, it requires manual resolution.

Variations of this command include git merge --no-commit <branchname> which merges the branch but does not create a new commit, and git merge --squash <branchname> which allows you to combine commits from the merged branch into one commit on the current branch.