Thread lock is a special polymer glue applied to fasteners before assembly. It dries in place and prevents the parts from coming apart until a certain critical torque is reached. It is very important for any environment with lots of vibration (like the front end of a bike). I am using blue threadlocker for the three adjustable stem top cap fasteners on my 2008 Felt B2 and it seems to be strong enough. Loctite is a famous brand name for thread lock. Just ask for "Loctite" or "thread locker" at your local hardware store and they'll know what you mean. There are stronger grades (like red) but I am hoping that blue is sufficient for this application. I also use it in a few other places, e.g. the stem face plate bolts.

One thing I am sure of is that the Felt assembly video doesn't mention using threadlocker which may be why these things are coming loose all over the place. Felt guys, are you listening? Would you want to ride a motorcycle that didn't have any threadlock on critical fasteners? I didn't think so :-) hahaha