No, you submit the data about your trip when you arrive in the US. The app will then display a pass containing a 2d barcode that will be valid for 4 hours. You'll scan that in front of the immigration officer and in front of customs officer. They still will ask you questions about your trip. It basically just makes your declaration electronic instead of filling out the paper form.

You would need to fill out the profile information before submitting the data, but that doesn't take too long.