This is actually an old one and I think the meaning is obvious: Don't waste your time chasing an opportunity you know you aren't going to win. Spend the time and resources on new accounts or on ones that have probabilities to close.
The problem with this, however, is when the account is so large and seemingly important that top management keeps telling you to go back. I have one of these now, and it's frustrating telling management that it is a waste of time and getting told to go back in there anyway. I guess this is just one of those frustrations that comes with the job.