Web sites have two audiences:
The first is human
The other "spiders" or "bots"
While the contents of the head tag displays little to human viewers it's important to the bots (also called web crawlers). Search engines have crawlers (e.g. Google's is called "Googlebot") that constantly surf the net following links from one site to another collecting and indexing everything they find. Google has an algorithm that then decides which pages are most relevant to search terms.
Web sites that read well to spiders are most likely to appear high in search engine results. This is what is called SEO or Search Engine Optimization. Books – big thick books – have been written on SEO. We don't have the time to go into it in great depth, but I will repeatedly touch on it as you proceed through the tutorial.
The single most important thing you can do for SEO is write a good title tag. My title tag for this page is:<title>Head tag - HTML5 tutorial</title>. It doesn't appear on the page itself but does show in the bar across the top of the browser and in the button in the taskbar, usually found at the bottom of the monitor on a PC
