API Question: What does this tool do? And why should I use it?

The API or Application Programming Interface makes it easy for computer programs to request and receive information in a format they can understand.  

If you are new to APIs, take a look at the Introductory Tutorial that walks you through step by step.  Below, you will find an Introduction to using the current (v2) API that covers the basics, along with the GET and POST calls and endpoints.

Finally, if you are an advanced API user, you go straight to the Endpoint Index and the Methods to access each.