That's why you have backups.

I feel like if your body follows the Unix filesystem structure, you have a real problem.

Acts as a wildcard for any directories that exist between arteries and clot.

It's a glob pattern (edit: tried to find a source that actually showed ** in use).

Had to look this up as well. Its not rm specific:

* is a simple, non-recursive wildcard representing zero or more characters which you can use for paths and file names. ** is a recursive wildcard that can only be used with paths, not file names.

More here.