What does '5.11' stand for you might ask. 5.11 tactical pants were originally designed for rock climbing, and '5.11' is a rock climbing level of difficulty. 5.0 is easy and 5.10 is difficult, so 5.11 is extremely difficult. An example is a move that is seemingly impossible, but occasionally there is someone who achieves it.
From here: http://www.police-information.co.uk/police-5.11.html