By definition, an application programming interface (API) is a set of routines, protocols, and tools for connecting software applications.
Simply put, in the vacation rental industry, an API is the communication protocol that allows your technology systems to connect and interact with each other. For example, think about your software system, website, and keyless lock system. Information in your property management software/system (PMS) is pushed out to your website and keyless lock system, and information from your website and keyless lock system is pushed back into your PMS.
This connectivity is accomplished via APIs, and each PMS has a different API specification. Your PMS company provides this API documentation to your other tech providers, and each of your other technology providers “writes to” this API to connect with your PMS.