In principle describes a basic idea. If your mother supports your travel plans in principle, she likes the idea of you getting out and seeing the world — though this could change when she sees the cost of the trip.
If you agree to something in principle, it means you're in favor of it based on what you know so far — the principle, or the idea, seems good to you. For example, while a lot of people support the idea of a new garbage dump in principle, few of them are happy to learn that it will be built near their homes.