What is an API?
API stands for Application Program Interface, which is a type of software that lets two applications talk to each other. Every time you use an app like Facebook or check the weather on your phone, you're using an API.
Most properties that work with Booking.com are also visible on other online travel agencies (OTAs). When a booking comes through one of those OTAs, an API allows the Booking.com system to interact with the other OTA's system and synchronize updated availability across all of them.