Let me start from the beginning: yes, the “installer” is only 2.6MB, but it will download a 310MB dmg with the actual game.
After that, let's get into the game. It's a point‘n’click adventure, with video scenes between the different enigmas. In this demo you'll explore the first cave, where you'll learn to start a fire and build your weapon. The characters are not so great, but the environment is actually pretty good, and it runs fine even on a 1st-gen MacBook (it lags only when you're outside, with wide landscapes to render).
It gives you some nice historic background too, if you want to explore it… you don't need to, but it can help.
I don't know the other parts of the game, but this is still a nice demo that will last for 15-30 minutes: the time depends only on your ability to solve the game, and how much you want to actually watch the beautiful paintings on the walls.