Definition: "Lay up" is a term that can be used as a noun, verb or adjective, but always describes the same thing: choosing to hit a shot shorter than you are capable of in order to avoid a hazard or to position the ball in a certain spot.
Let's say you've hit your tee shot on a par 4. You've got 200 yards to the green. But there's a creek running across the fairway right in front of the green. You're just not sure you can clear that creek, since you've got 200 yards to go to get to the green.
So instead of attempting that risky shot, you decided to "lay up" in front of the creek. Rather that taking a long iron or fairway wood for a 200-yard shot, you might choose instead to play a short iron or wedge and hit the ball around, say, 130 yards. That lay up shot would leave you a very manageable 70 yards to the green, a short shot that likely takes the water out of play.
Lay up shots are a common factor in good course management.
