The Mendix Runtime executes the model created in the Modeler. It serves pages to the Client and executes microflows, calls web services, generates documents, communicates with the database and much more.
You need a license to run an application in production mode. Without a license the Mendix Runtime stops operating after a couple of hours. A license can be obtained through your account manager at Mendix. Afterwards, you can activate your license by following the activation manual.
You can extend the functionality of the runtime by writing Java actions. The API available for Java programmers is available at https://apidocs.mendix.com/4/runtime/.
The runtime logs activity at different levels. You can read more about logging and log levels at the Logging page.