What this seems to suggest is that you are using a google map API key that will work with *any* application (unrestricted) - meaning that if someone were able to extract the API key from your app, they could use it in *any* other app, and if their API usage is excessive, that unauthorized usage can be charged to YOUR account, because the API is registered to you.
So what this message is doing is suggesting that you should restrict the API key to just YOUR app, so even if someone else finds out the API key, they won't be able to use it in any other app.