Finesse is a ratings-based stat that will directly reduce the Resistance of monsters as well as their Block/Parry/Evade ratings. This stat will appear primarily on instance/raid/pvmp loot but there will be a few pieces available with Finesse from quests and crafting.
This new stat is a two sided weapon because monsters will have access to it also. Most landscape monsters won’t have much if any Finesse but expect high difficulty instance/raid Boss monsters to have quite a bit.
Note that you can now buy the skirmish personal stat finesse and slot it, even at low level.
Finesse rating was added with the Rise of Isengard expansion.