Q: What does API mean?

An “Application Program Interface” (API) is is a contract that GeoTools commits to using to make functionality available to your application.

The interfaces in gt-opengis and gt-api represent the formal GeoTools interfaces advertises to your application defining concepts ideas and data structures.

Additional public methods and classes are added by other modules, but they are focused on doing rather than defining.

For more information review welcome How to Use GeoTools page.