Thread locker is an anaerobic adhesive applied to the threads of fasteners before assembly. It cures into a hard plastic only when deprived of air (i.e., when the bolt is tightened). Its purpose is to lock the bolt in place, preventing it from vibrating loose due to engine or road vibration. It comes in different strengths: Blue is “medium strength” (removable with hand tools), while Red is “high strength” (requires heat to remove).