Slenderman is a myth, he would be a tall man with long arm and no face that would have appeared on photo's. Slender Wiki and Slender Myth

The game of Slenderman leaves you in a dark forest with only a flashlight. You need to walk around (doesn't go fast, as you are a walking human) with a flashlight on batteries, that will go less and less bright after time. In the forest you need to walk around and search for 8 pages. But watch out, Slenderman is around...

A funny video about the gameplay: