Spool holder
This is the pin at the top of the electric sewing machine where you place the cylindrical plastic reel that carries the thread.
It's useful to have a couple of spool end caps of different diameters – these are the plastic discs that hold the reel in place and help to feed the thread.
Some sewing machines have two spool holders, so you can sew with two different-coloured threads at the same time with a twin needle. That's a bit advanced for me, so that tips is a little redundant for those people who probably know what that's for anyway...:)