For detailed information, you can follow the "What is an Open API" YouTube link for a video that explains the definition.
The term "API" stands for Application Programming Interface, but it might as well stand for "(A)nything's (P)ossible with (I)mportOmatic!" An API allows users to write code to interact with ImportOmatic as it's running (even if you're nontechnical, don't worry: there's still something in this for you so keep reading!).
So what can IOM do with an API? How about automatically emailing a solicitor or team captain if one of their relationships makes a gift? How about automatically updating or retrieving information from an external database? (Acquisition lists anyone?)
How about reaching out to a webservice or FTP folder to automatically grab the data or file that is to be imported (rather than downloading a file manually)? How about separating a First Name value like "Bob & Sue" and making it into two different records? With an API, there is hardly anything that can't be done with your imported data.