A marker is something that's used for many different things - it can be used to show where the beginning of a round is (in circular knitting), it can be used for where you need to make decreases/increases on subsequent rows, etc. The marker itself can be anything from a loop made from scrap yarn, to fancy beaded beautiful pieces, that all serve the same purposes. All you do is slip it onto your needle where your pattern tells you to, and when you need to "slip marker" all you do is move it from your left needle to your right needle when you come to it.
As for your second question, whenever you see numbers inside brackets like you described, it is referring to the different sizes of the item. For example, if a sweater were made in four different sizes it could say something like S (M,L,XL) and then the corresponding numbers refer to those sizes.