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. 

Get a Key/Authentication

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 beetween = 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.