Used by WEB applications to access your hosted databases. An API KEY will associate a storage location and database structures that can be used later by a WEB service for read/write operations.
One account can have more than one API key.
The blogging application uses one NoSql table to store its articles/post lists. If you will have more than one blog you will need more API KEYS accordingly, as the blogging applications will create and manage the data using one API KEY.