This signal is just a warning sign. It's information about a possible danger.
You're not supposed to do anything in particular as rule. You should assume a defensive position against this event though. Whatever that means it's up to you. Locals may know about it, but you, that come from a flat land, wouldn't even suspect. There are many of these signs, that I learned in driving school, and I never saw in real life. Not all dangers are everywhere. Sometimes, when travelling one pops.
You might think it's very useless but take this example:
Imagine I aproach you and I slap you by surprise in the face. What would your reaction be? You would be completly surprised - you could run away afraid or you could even react in surprise and do the same. But I did slap you. Or maybe I just caught a mutant bug in your face that was about to kill you. You don't know!
Now, imagine someone warns you I can slap and I approach you with the same intention? what would you do? you would be prepared and could react properly!
It's the same here. You will have a more defensive attitude towards falling rocks or rocks on the road. You should assume both situations are possible. If at night, with wind or rain you will probably want to drive slower or you might even drive at the same speed if confident enough, but you will be aware of the rocks. If some fall or you hit one with your car you know what happened and will probably react in a more appropriate way. If caught by surprise you could react in unexpected ways and cause a larger accident by driving off road or against another car.
To conclude: this kind of information is very important to make you aware of possible dangers that otherwise you wouldn't know of. You should than adapt your behaviour accordingly to the risk, considering external factors as well like wind, rain, visibility and traffic conditions, but it's up to you to decide.
If you're puzzled with that sign you should look at this one, it's my favorite:
This is also a danger sign, and basically means "various dangers". You should basically prepare... anything can happen! :)