You will get to learn all the jargon soon, but it actually goes back to days when you would write your flying request on a piece of paper.

"While slip" (WS) is to pick up a trip at straight pay
"Green slip" (GS) is at double pay for a line holder. (Not double for a reserve, but over the guarantee)
"Yellow slip"(YS) is for a reserve and is used for various things, but for the most part is to be "out first to fly" or used to pick a certain trip that he may desire.