When used with a tool and posts this wire makes semi-permanent connections that can be quickly added & removed. The wire needs to be malleable enough to tightly wrap around the post but rigid enough to not unravel. Generally it will be 30 AWG silver-plated single-strand copper wire with Kynar insulation.

This is very old technology, that's still quite useful today. It was used before etching your own PCBs became popular.