A “réponse de principe” is a non-definitive answer, i.e. what you would answer to the best of your knowledge right now. Let’s say for example group of friends are going to a beach house for a week in August. You meet the organiser at a party and he asks you if you think you’ll come. Of course, she can’t expect you to be able to give a definitive answer immediately: you’d have to check your agenda and possibly discuss it with your partner before accepting.

Thus she asks for a “réponse de principe”: you could say “I’ll probably come, but I still need to check the details. In the mean time, you can count me as a participant, I’ll give you my definitive answer in a few days.”