It's a warning sign alerting you to the possibility of falling rocks.
Yes, they are often netted with steel mesh and have 'catch areas' to contain the fallen rock.
It's really no different than 'construction ahead', 'steep grade ahead' or any other warning sign. It just lets you know of a situation on the road ahead.
It does not mean the rocks will fall, just that the possibility exists.
I've seen signs near airports warning of ow flying aircraft. There were no aircraft I could see. The sign just means there could be low flying aircraft.
It's really a simple, self explanatory sign. Don't read too much into its meaning.