Reservations Tab allows you to view all the reservations you have, from all sources. This includes all past, present and future reservations.
This can be used to filter the sources of guests' reservations. This means you can search for guests based on what website they've booked from.
For instance, I want to see all Booking.com bookings:
1. Open reservations
2. In all sources at the top of the page, click Booking.com
3. Hit Search and Voila (see results below)
Click on the booking to open up the reservation details to modify any existing information