A spool cap is the piece that attaches to the spool pin, that holds the thread in place and helps the thread properly unwind and not jump around while you're sewing.
We stock many types of sewing part for sewing machines. Even if you don't find the one you are looking for, it does not mean we don't have it in stock. Please let us know machine builders' reference numbers and/or machine make & model and we will find one for you.