Bonafide, real, and documented visually or with x-ray. The reason I am “watching” it rather than filling it is that current knowledge provides a reasonable opportunity to stop the progress of the decay with proper daily preventive care.
There is even the possibility of reversing the decay process to the point that the lesion will disappear on subsequent x-rays. The cavity would be “healed”.
Clearly, reversing a cavity, or even stopping a cavity that has already started, will take greater diligence than what has allowed the cavity to start. This begins with better cleaning routines. The huge majority of these watch areas must be cleaned with floss every day. If that is not happening, the decay causing bacteria will win and the cavity will grow, eventually necessitating a filling. Additionally, the decay causing bacteria will be starting other cavities in new places. Therefore, floss floss floss!
Remineralizing, or "healing" the cavity, takes more than just keeping it clean. Fluoride is the best remineralizing tool we have. The primary action of fluoride occurs topically on teeth. We don't have to swallow the fluoride. Therefore topically applied agents like toothpaste, fluoride rinses, prescription home fluoride treatments, and professionally applied fluorides are what works best.
Please remember, watch areas are real cavities in progress. If they are not cared for diligently, they will do what all cavities do. They will get larger and need treatment.