This acronym stands for:
Previews Of Coming Distractors.
This is a type of distractor which happens before the session begins, and can interfere with the session without you realizing it.
For example, let's say that you are expecting a phone call and know that it could interrupt the session. If you don't set the worry about it aside, then you will spend the entire session with part of your mind waiting for the phone to ring.
This is also referred to as "Physical Inclemencies", even though it may not always be physical - for example, worries about finances, etc. can act as P.O.C.D.s, as well.