I believe that reduced effectiveness means it reduces the strength of the snare.
However, it reduces the % multiplicably, not additionally. So if someone slaps a 50% snare on you, and you have 15% snare reduction, the snare wont then be 35%, it would be 42.5%.
I have never tested 100% snare reduction, like a redguard light armored warden wearing rangers for example. I have no idea if you can actually become essentially immune to snares.
There is a set that reduces snare duration instead of snare effectiveness. That set is called barkskin and it is actually pretty decent.