Meaning
A spool of thread, usually red, with a loose end hanging free. It stands for the basic material of tailoring and handicrafts.
It covers sewing, mending, embroidery and handmade crafts: “Fixing my favourite shirt tonight 🧵” Online it is also a common marker for a long multi-post thread, signalling that a story or explanation continues below. It can hint at a common thread linking ideas too.