Google will count each time your API key is used on your site. If you go over 25,000, you will need to start paying. When placing your API key on your site, it is visible to others who can simply copy it and use it on their own site and have you pay for their use out of your quota.
To stop them, you simply tell Google that your key can only be used on your site (what is called an HTTP refferer). All you have to do is visit the Google API console, select your project and your key and set:
- Key restrictions: HTTP referrers (web sites)
A text box will appear:
- Accept requests from these HTTP referrers (websites) (optionals)
You will need to place the domain of your blog with a *. before and /* after . This simply means that it can be used on any page on your site. So if your domain is travel-blog.com then you should use:
Don't forget to hit the Save button.