When you see the sign, what does it mean?
Well, it’s actually a warning that there’s a hiker ahead. In some cases, it could mean that there are rocks on the trail that you should be careful about tripping over. But I think more often than not, it means to just be careful because there’s a hiker in front of you.
You can also use this sign when you want to warn other hikers that you are hiking up or down the trail. It is usually used by hikers who don’t want to disturb other hikers by making them aware of their presence.
This is similar to how cars have lights on their headlights so others can see them. 
Also, there are times when I hike alone and come across other people who are hiking together. I would put this sign out to let them know that I am hiking by myself and they should be careful because they might trip over me as they go down the hill.