Bondo is a sandable automotive body filler. It's a 2 part material similar to resin. You have the base putty, and cream hardener. You mix it in the ratio according to the instructions on the can. It's applied to Pep parts in areas where you want to smooth out or fix the shape of. You apply it in light coats, sand smooth when dried, and you repeat this process until the part has the desired finish.
Remember, Google and YouTube are you friend!
Bondo Wiki for more info
There are plenty of instructional videos and tutorials on YouTube, all you have to do is search for them. Hope this helps!
Oh, and you can find it at just about any automotive store, as well as Wal-Mart, Lowes, and Home Depot.