Why does `git branch` annotate a branch with `+` and another with `*`?

When I do git branch, it returns something like:

~/Dev/project/main (main) $ git branch
  bug22
* main
+ feature33
  feature19

This is because you're using git worktrees.

• * Whether or not you're using worktrees, * (always) annotates the current branch you've checked out.
• + Annotates the branch that's checked out at your other worktrees.

like if I tried checking out feature33 then I would have got the following error:

fatal: 'feature33' is already checked out at '~/Dev/project/features'