Definition of close
Close is a word that means "close". To put it in a poker situation, we would use the word to refer to a situation that is "close" between two actions (one option is worth another). Often, in poker, the best options are easy to determine. Four players are all in before me, I have a mismatched 2 and 7, I have the option to call or fold. Here I fold and it's an easy decision, it's not a closed decision. A closed decision can also be called a "break even" decision or a decision that will have a neutral EV.
Example of a closed decision
You are on the river and have a straight. You are in position against a very skilled player. On the river, a third heart has come up. You don't have the flush, just a straight, but you feel you have the best hand at a high enough frequency to bet. You bet and your opponent check-raises you. You have notes on him and you know that he is well balanced, that is, he sometimes makes this play in a bluff, sometimes because he has the flush. Should you call the bet? The decision is closed!