Now on to the code itself:
<!-- Ethnio Activation Code -->
- <!-- Ethnio Activation Code--> - Just a comment explaining that the code below is the Activation Code. Doesn't actually do anything to your page.
- src="//ethn.io/xxxxx" - the // allows you to place it on a SSL or regular HTTP page without changing the code. It cleverly assigns HTTPS or HTTP dynamically.
- async="true" - This just tells the browser to load everything else before it loads the Ethnio code.