Dining rotations are which dining room you eat in each night. THere are 3 rotations, each beginning night 1 in a separate dining room.
IMHO, rotations are an overrated thing to worry about. By the end of the cruise, each guest will have experienced each restaurant twice and one 3 times. Each will have eaten the same menus. Some people like to be in the fanciest restaurant for formal night and the most tropically themed for pirate night--reality is that you'll have the same food regardless of where you are seated. Other people want to know their rotation in order to schedule adult dining on a particular night (to miss a particular menu).
If you've never cruised, I don't know why you would feel the need to make a request. 20+ cruises later, we request a server--we are happy wherever he is.