The Inspector window appears when you add a new event, or double-click on an existing event in the Calendar app in macOS Sierra. It shows you all the details of your events, including who's invited, and where and when you want the event to occur.
It helps you fill out your events using Autocomplete. Start typing an address and Calendar suggests matching locations. Start typing the name of a contact in your Address Book and Calendar suggests matching names based on contacts you've entered in the Contacts app.
What’s more, you can see your event locations on a miniature map in the Inspector. If you click this mini map, a full size view opens in the Maps app.
The mini map includes an estimate of the travel time to your appointment destination from your current location in the event Inspector as well as the weather forecast for that day. (For travel time estimates, directions and weather, you need to turn on Location Services in System Preferences.)
(This how-to is based on my experiences and info on Apple's support pages — where the images come from.)