It's just some kind of damage. It's not a bad looking stamp to start learning with, but it isn't worth much. They go for a couple of buck on eBay
. Feel free to ask questions as you start going through them. I'd suggest getting a Scott catalog. You can get one from a few years back quite cheaply on eBay
and sometimes they have them at libraries. Try to get things organized and I'm sure more questions will come up. Banknotes (136 is one of them) have several varieties and there is plenty to learn to start sorting them out.