If you just purchased a soldering iron you may have one that comes with both a sponge and something that looks like a kitchen scrubby. These are provided for cleaning the tip of your iron.
So which to use?
The brass scouring pad will clean off the tip much better than the sponge. Also, if you are soldering for a while you will find the wet sponge will drop the temperature of the tip, so the brass pad will keep the tip evenly heated.