This is a spool with 40 feet of yellow 20 AWG stranded copper hook-up wire. It conforms to the UL1007 specification and is rated for up to 80°C and 300 VAC.
Stranded wire is more flexible than solid wire and is good for projects where you need the wire to be able to bend. The wire ships on small plastic spools in lengths of 40 feet. One side of the spool includes a tab with a slot that can be used for hanging.