Explanation

Box 1: Event handler
An event handler is the place where the event is sent. The handler takes some further action to process the event.
Box 2: Event subscription
Event subscriptions - The endpoint or built-in mechanism to route events, sometimes to more than one handler. Subscriptions are also used by handlers to intelligently filter incoming events.
Reference:
https://docs.microsoft.com/en-us/azure/event-grid/event-handlers
https://docs.microsoft.com/en-us/azure/event-grid/overview