"Tweener" means a hole that has a scoring average in between 2 numbers, like a hole that averages 3.5, for example. Almost all scores are one of two numbers: 3 & 4 in this case. A wide open 600' hole might be a tweener for advanced players. When a hole must be wide open, tweener holes can be a good thing, because 2 scores are produced often rather than just 1.
People confuse this with a hole an "auto-par" hole, where just 1 score is achieved. A 480' wide open hole would be too long to birdie and too short to bogey for most advanced level players, for example. Scores would just be 3's, as if the hole wasn't played.