Burnt flux and oxidized material can accumulate on the tip of your soldering iron causing heat transfer to be blocked. It can also contaminate the joint, therefore making soldering very difficult.

When you buy a new soldering iron it often comes with a sponge which if you dampen can be used to wipe clean the tip of the iron (when hot). Wet denim can do the same thing.

A small amount of fresh solder is usually then applied to the clean tip in a process called tinning.

The tip should be kept tinned (coated with wet solder) to minimise oxidation. Oxidation can block heat transfer, corrode the tip of the soldering iron and contaminate the joint.