When there's a debate or discussion going on, a "good point" is an idea which is convincing and makes sense.
A: I think we should take Highway 75.
B: But I bet there's a lot of traffic right now.
A: That's a good point.
You can also just say "Good point" when someone says something convincing:
A: There's no way we'll be able to finish in time to make it to the opening.
B: Good point.