When there's a debate or discussion going on, a "good point" is an idea which is convincing and makes sense.

For example:

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.