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When someone says 88% of AT, what are they talking about?

Aerobic or Anaerobic threshold most likely

I believe AT is anaerobic threshold and AeT is aerobic threshold--but I could be wrong.

Yes, AT means anaerobic threshold. Most people now say LT for lactate threshold, since your body produces more lactate as heart rate increases and the anaerobic processes begin to dominate over the aerobic.

So when someone says 88% of your AT or LT, that means heart rate. If your LT was a heart rate of 160, then 88% would be a HR of 141. Most folks train with heart rate monitors, so one sees a lot of that sort of talk.

spot on.

Well actually its the threshold when you move from aerobic training to anaerobic. Its the dividing line. You're always aerobic (whether sleeping, sittting, running etc.) till you go into O2 debt, at which time you're anaerobic