A static API is simply a collection of flat JSON files that live on a content delivery network (CDN). It doesn’t perform any action other than delivering content (static JSON files) to the requesting user.
But that doesn’t mean a static API is simple. And every file doesn’t have to be manually generated or updated. A static API can still use a database and it can still pull data from external services. In other words, it can be dynamically generated, but it is statically delivered.