what he meant by that is:
duskblade has a passive component where if the enemy team hasn't seen you on the minimap for 1 seconds (or more), your next auto attack (or skill that acts as auto attacks, like ezreal's Q, nasus's q, etc) will apply bonus damage and slow the target for 99% for like 0.5 seconds.
So, you see that small indicator at the same place you see how much time left your red buff has, or blue buff, or your eve stealth indication or your twich stealth indicator, or how much you have until you can use that Q as nasus to auto attack something before it goes on cooldown.
That's why, you can, at all times know if you are seen by the enemy team or not. Hence, know if something is warded or not. If you have no duskblade passive damage icon = warded.
If your duskblade passive damage icon starts counting down (like twich q, nasus q, etc), then you just stepped over a warded area or in the vision of the enemy.
Which is quite broken as you don't care about the 'cooldown' on it. It might as well have 0 cd, sure you can't destroy the ward like you could with the passive that reveals wards or sweeper.
You know if the area is warded or not :)
Which is a very big advantage for AD cast, like they need more looool.
Ofc, in bronze or silver people that play ad assassins might not even know about this, meh, you might even stand on wards and they won't always notice due to not looking on the minimap. But the higher you go, the stronger that shit is. Knowing if you are seen or not is a big deal