Tinning is the process of coating something with tin. In electronics you tin the tip of your soldering iron to prevent corrosion or you can tin wires so that they are easier to use.
One common use of tinning is to tin wires. If you were making a project and only had stranded wire instead of solid core wire, you would want to tin the ends to make it easier to use. It is much easier to insert a tinned stranded wire through a hole than one that hasn't been tinned. This also helps prevents loose strands causing a short circuit later.
Tinning Your Soldering Iron Tip
After soldering for a while your soldering iron tip may get rusty or have black spots on it. This means the tin coating is starting to be removed and you should tin your tip.
After cleaning the tip of your soldering iron you want to add a small amount of solder to the tip to help protect it. Wipe off any excess on a damp sponge or a brass tip cleaner.
Make sure you use rosin core solder as the rosin or flux helps the solder adhere to the iron. This is known as tinning the tip or just tinning.
Tinning makes your tip last longer and makes it easier to solder.