Quiet. No sound from the instrument.
It means to mute that string, or to not play it at all.
It Stands for the pickup arrangement on the guitar; H- Humbucker S- Single coil S- Single coil :)
I believe this means that the length of the strings between the nut and the bridge is 24.75 inches.
I always go to ultimate-guitar.com, they have great tabs.
It stands for "blemished" and it is a discounted product usually It could be from a return from a customer being resold by a store or it could be a small, usually cosmetic, defect from the factory.