Google API keys - What is server key and browser key

I ma using google maps in my app, and use the URL as following format without the API key- values,

<script type="text/javascript"
      src="https://maps.googleapis.com/maps/api/js?sensor=SET_TO_TRUE_OR_FALSE">
    </script>

Now I need to use the API key to track the requests, according to the examples it needs to add another API key query string parameter,

<script type="text/javascript"
      src="https://maps.googleapis.com/maps/api/js?key=API_KEY&sensor=SET_TO_TRUE_OR_FALSE">
</script>

But, I received 2 keys, One for servers (IP address restrictions) and one for browser apps (domain level restrictions) are as follows:

Server Key: XXXX Browser Key: YYYYY

Which key should I use in my app, should I use both of them? Could not find a good article on this.