It's just a dps version of blade barrier, except for all your runes instead of just the blood ones.
"On cooldown" means you have used them and are waiting for them to become available again. For example, on an AoE pull, you pop DnD, IT, PS, then Pestilence, and now you have 3% increased damage.
Thx for explaining what cooldown is Winlol
If only I had known. I thought the DK DPS T10 4-piece bonus was shite but you must be the Yoda of DK's. Thanks for enlightening me, now I can't wait to get the full set and just go out there and blow all my runes ASAP.