Nod back. It's an acknowledgement of being a fellow motorcyclist/biker. we're not like confirmed car drivers, if you break down, just pretend we havent seen you etc.
Obviously only nod/ acknowledge with your hand/ flash your light but only if it's safe to do so. Dont try it banked over in a bend etc.
Or if on this site, go into profile of who nodded you and press 'NOD' same effect.
Does that help ?