Yep, many of us do know what that means. It is a chit sheet, (I believe that is what they call it). It tells the DI what they cannot do because they are or were sick. Mostly, just not drinking enough water.
You'll hear that term a lot in the future. It can be anything from a sick chit to a special liberty chit, to an attendance chit, etc etc. Basically just think of a chit as a piece of paper on which things are usually routed to other people.
So for the sick chits, it's from the doctor to that person's chain of command.
I saw my son with a white band last year as a plebe and found out it was poison ivy. I guess the plant is along the trails they run. He is very susceptible to it and got treatment right away. He also picked up ring worm during the third tri when he was in the wrestling room working out a few days (not a wrestler). Just a head's up if you have a DS or DD who is susceptible too.