Fairing a surface means to smooth it out. This can mean filling low spots or holes, or sanding/grinding down bumps until the surface is level. The material you use to fill or reshape is called a fairing compound (fairing putty).
Fairing compounds contain some type of resin — epoxy, polyester, or vinyl ester. For convenience, there are premixed fairing compounds such as TotalBoat TotalFair or TotalBoat Polyester Fairing Compound. You can also make your own putty by combining resin (epoxy or polyester resin) with silica thickener and microballoons until the mixture is the consistency of smooth peanut butter. The microballoons will make the material easier to sand smooth, once cured.
Note that, if using an epoxy system to make your own fairing material, be sure to mix the resin and hardener thoroughly before adding fillers/thickeners; if using polyester resin, add the fillers/thickeners before catalyzing with MEKP.