The idea behind spoke prep is both a lubricant and thread-locker in one. It acts as a lubricant during the building process and helps to prevent binding between the nipple and spokes as tension increases.
As binding happens, the spoke itself begins to twist which will cause uneven tension your new wheel causing truing issues both before and after it's first few uses.
Perhaps the most renowned spoke prep is called, "Spoke Prep" from Wheelsmith. Don't get too confused by the color options of blue and gold. They offer both colors as a way to ID drive-side and non-drive side spokes for the build, but function of both colors are the same.
Are there alternatives to spoke prep? I'm sure there are, I've heard of people using Locktite, Chainlube, Linseed Oil, etc. My opinion, Spoke Prep is designed for one thing and one thing only, to help you build and tension your wheels properly and few extra bucks to do it correctly is worth the price of admission.
Wheelsmith Spoke Prep"The ultimate spoke thread compound is SpokePrep™ because it is both a lubricant and a thread lock! How can it do both? When spoke tension is low and there is risk of loosening, SpokePrep "locks" the nipple by maintaining a minimum nipple torque. When spoke tension is high SpokePrep keeps nipple torque low with Teflon lubrication. We also have SpokePrep in two colors so you can prep your spokes and immediately know on which side of the wheel they will be used."