I researched your question and found that API enables two software components to communicate with each other. MLS API or (Application Programming Interface) is a way to display property listings online. For members, check on HAR, you can find IDX Tools And Widgets under https://cms.har.com/idxtools/. Like other IDX technologies, MLS API allows you to connect to MLS, build a property search that updates with new listings, and generate leads. IDX is a set of data approved for public display. I hope this helps!
source:
https://cms.har.com/idxtools/ and google searches for definition of API
After a little research I found that An API (also known as an Application Programming Interface) transfers real estate data from a third-party. MLS API is a way to display property listings online. Like other IDX technologies, MLS API allows you to connect to MLS, build a property search that updates with new listings and generate leads. I believe if you go to the cms.hard.com/idxtools page in HAR.COm, you will see a list of tools and widgets which offer to online consumers to enhance and generate leads. I hope this helps.
source:
cms.hard.com/idxtools/ and google searches