A direct booking is a reservation that a guest makes directly with you, via phone or from your business website/booking engine. The Availability Online Booking Engine is a direct booking engine that can be added to any website platform.
A non-direct booking is a reservation that a guest makes with a third-party organization, best known as Online Travel Agencies ( OTAs ). Examples of some OTAs would be Booking.com, TripAdvsor, VRBO, AirBnB, etc. There is no debate that OTAs can be beneficial in increasing reservations, but these third parties do take a sizable cut out of your revenue.