API stands for Application Programming Interface, which is a type of software that allows two applications to talk to each other. Every time you use an app like Facebook or check the weather on your phone, you’re using an API.
For example, most properties that work with Booking.com are also visible on other online travel agencies (OTAs). When a booking comes through on one of those OTAs, it’s an API that allows the Booking.com system to interact with other OTAs’ systems and update availability across all of them in sync.