Tinning a soldering iron or rather the tip is something you should get into the habit of doing regularly. If you keep your soldering iron tip tinned it will last longer and be better at soldering as it will conduct heat better.
Soldering iron tips are predominantly made of copper. This is because copper is excellent at conducting heat and that's exactly what we want it to do. Unfortunately if they were just made of copper they would very quickly dissolve with the application of solder. To stop this happening they are iron plated. This is only a thin layer though and you need to be careful when using anything even mildly abrasive when cleaning the tip.
If a tip oxidises or burnt flux accumulates it won't be able to solder properly cleaning the tip frequently will also prevent the build up of harder to clear deposits that can become burnt on. A dirty tip wont “tin” very easily and hence won’t solder very well either so I think we are all agreed its best to keep your tip clean.
If a soldering iron tip is particularly black and dirty it can sometimes be bought to life by using a sal ammoniac tinning block. This should only be done in a well ventilated area because of the fumes it gives off. Basically you just gently rub and rotate the hot soldering iron tip in the block. It will start to smoke and melt into the block and at this point you should introduce solder to the tip. This should take to the tip much better given the addition of the sal ammoniac and you will have a shinny looking tip again.