I'm not sure what that person meant by "listening to your sleeve" but this is how I would interpret it. The restriction in your tummy will tell you when to stop eating. The feeling of being "full" can be very different post-op, so you have to learn the signals that indicate you are approaching being full. One time, shortly after surgery, I overate because I didn't recognize that I was full. It was a miserable experience, I was in severe distress until finally I threw up. After that, I learned how to identify when I've had enough to eat (and honestly, I rarely eat enough now to test my restriction).
Also, your sleeve can educate you in terms of WHAT you are eating. My new tummy does not do well with red meat. Regardless of how prepared, red meat is uncomfortable for my body to digest. I could keep trying to force it but I've chosen to "listen to my sleeve" and just leave red meat out of my diet.
Your body will give you signals about what it does and doesn't like and they are worth paying attention to.