This is a water soluble liquid flux pen. A flux pen is a must have when performing any kind of solder rework. The flux is the substance that prevents beading of the solder and helps the solder flow cleanly onto the parts you are soldering. However, flux is mildly corrosive over long lengths of time (months). The water soluble flux will cleanly wipe away after wetting the soldered surface.
Check out Sparkfuns Surface Mount Soldering Tutorials for some great soldering examples!
Be sure to use in a well ventilated area!