The Unite Developer Hub is a website with all the information you need about ON!Track Unite and how developers can build and manage connections with ON!Track. You can visit it at ON!Track Unite Developer Hub.
If you want to know how to connect ON!Track Unite with other software, please check: How can I connect ON!Track with other software, and what are my options if my software isn’t supported?
If you want to know what Unite is, please check: What is ON!Track Unite?
If you want to know what is APIs are, please check: What are APIs?