Definition of close
close is an English word meaning "close". To put it in a situation of poker, we will use the word to talk about 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 2 and a 7 that are out of phase, I have the option to call or fold. Here, I fold and it's a decision that is easy, it's not a decision close, you can also say of a decision close that it's a decision "break even" or a decision that will have a neutral EV.
Example of a decision close
You are on the river and you have a straight. You are in position against a very skilled player. On the river, a third heart has shown up. You don't have a flush, just a straight, but you judge that you have the best hand at a high enough frequency to bet. You bet and your opponent bets you check-raise. You have notes on him and you know that he is well balanced, that is, he sometimes makes this game in bluff, sometimes because he has the flush. Should you call the bet? The decision is close!