All it does is sort your inventory, replace your broken tools/ weapons, and swap out / refill stacks in your hand from your backpack
it also sorts chests.
I think the default is "R", so say like I have 66 cobblestone, and I am building a wall. Two cobble in hand, and 64 in my backpack. If I use those 2 cobble in my hand, it will auto refill my hand with the 64 that were in my backpack, so I don't have to open my pack and do it manually. Also, in the same scenario, if I hit R it would then take 62 from the stack of 64 in my pack, and place them in my hand.
It is such a basic mod, but it is so helpful. I can't play without it.
http://inventory-tweaks.readthedocs.org/en/latest/
There is also a video somehwere that better shows it