A spool is a round object with flat sides and a hole in the middle. It is used to hold thread, wire, or film.
Spools are usually used for sewing or for holding things neatly.
Examples:
She put the thread on the spool.
The film is wound on a spool.
The wire is wrapped around the spool.
The spool keeps the thread from getting tangled.
I have a big spool of thread.
The spool is round and smooth.