Tinning, explained 

By using a soldering iron to add rosin-core solder to the wire strands, the solder fills in the voids between the wire strands and causes the loose, fine wires to become one solid wire that can be easily bent and placed underneath the terminal screws.

Tutorial - Tinning Electrical wire
1. What you'll need

Tinning Electrical Wire Tutorial
3.To tin a wire, apply the tip of your iron to the wire for a second or two, then apply the solder to the wire.

Tinning - Electrical wire
4.The solder should flow freely into the stranded wire and fill the wire.