Hello,
Greetings from Potenza.
As per Google recent announcement, usage of the Google Maps APIs now requires a key. If you are using the Google Maps API on localhost or your domain was not active prior to June 22nd, 2016, it will require a key going forward. Please see the Google Maps APIs documentation to get a key and add it to your website.
Visit below link and follow the introduction there. and create your API key.
After getting your key add it in your JavaScript. See example below:
<script async defer src="https://maps.googleapis.com/maps/api/js?key=YOUR_API_KEY&callback=initMap" type="text/javascript"></script>
Note: Please add your key between = and & sign
Hope this will help.
For more information kindly visit below links. this will help you to know more about API and google map.
http://stackoverflow.com/questions/37991340/error-google-maps-api-error-missingkeymaperror
http://googlegeodevelopers.blogspot.in/2016/06/building-for-scale-updates-to-google.html