as already posted its Damage Per Second, and its a way of calucating how good your gun is once you've applied all your gear to it.
Each gun will have a base damage, like 650. This then gets put into an equation along with a whole load of other stuff to work out how much damage your loadout does, then divides it by time to get the damage done per second
https://www.reddit.com/r/thedivision/comments/48cojz/the_complete_dps_formula_including_the_very/
This game is going to be very much like destiny in that the best players will know how to use the loadouts to get the upper hand. You aren't going to fare well in PVP if you don't know how to get the best setup and understand the difference between Firearms, Stamina and Technology and just equip any random armor or guns you find off the floor
If you don't know the difference between Stamina, Firearms and Technology, watch this.