Sliding down a ladder is a fast way to descend a ladder. Many modern games have this feature; this allows the action to be uninterrupted even when climbing down a ladder.
Usually a character slides down a ladder by putting both hands and feet on the outside of the ladder and letting themselves smoothly slide down it. This is realistic when the character is wearing gloves, however, it can seem ridiculous when said character is not wearing gloves. In real life, the metal from the ladder would heat up very quickly due to friction caused by the hand sliding on the metal. The character's skin would start burning, inflicting extreme pain.