Google official statement:
"As of June 22, 2016 we are making the following changes to the Google Maps APIs Standard Plan: We no longer support keyless access (any request that doesn't include an API key). Future product updates are only available for requests made with an API key. API keys allow us to contact developers when required and help us identify misbehaving implementations."
You can read more on their blog post here.
What is the reflection in Sitefinity?
Google Maps is integrated as part of the Address field in Sitefinity. For more information, see http://docs.sitefinity.com/administration-set-up-google-maps
How to obtain an API Key?
It's more than easy!
You need to create a browser key. Google has a page describing how to get the API key. If you click the 'Get a Key' button it will take you through the right steps.
First you will be asked to select a project or create a new one. Click 'Create a project' and add some meaningful name.
Then, you are redirected to the following screen:
Note that it's better to restrict your browser key by domain. If the key is unrestricted, other domains can make use of your key, which will reduce your quota allowances.
To restrict the key, click 'Go to Console' and click on 'Credentials'. Then, select HTTP referrers (websites) as shown in the image below and fill up the domain:
At this point you can go back to the Sitefinity backend and follow the steps in the Sitefinity documentation here to Set Up Google Maps.