What does an atrrs wait status mean?

It means what it says - you're on the wait list until a slot opens up.

Since you know you're on the wait list, if there's any prerequisites you can get done, do them. Example - I got put on the wait list for SLC Oct 2017...I got the welcome letter for the Jan 2018 class a couple weeks before the class started. However, since I knew I was on the wait list, I got all my personal affairs lined up (single parent and my daughter was waiting for a transplant) in case I got notified. Luckily I had that because there were a lot of people in my class who got notified a week before the class started. Some on block leave. Some deployed. So I was prepared more than some of them.

From my understanding, it means you are waiting for a slot. You do not currently have a definite slot to go.

It means exactly that. You're on a waitlist til a slot opens for you. Try and knock out all pre-reqs and prepare as if you are, but depending on how soon the course is, don't get super bummed if you don't get the slot. Happened to me with reclass, and they said ATRRS could notify me up to 48 hours from the class start date. I knew I wasn't going to get that slot but prepared as if I were anyway.