This is the page title that Google and most other search engines show in search results. Titles are critical to giving users a quick insight into the content of a result and why it’s relevant to their query. It’s often the primary piece of information used to decide which result to click on, so it’s important to use high-quality titles on your web pages.
The meta description summarizes the page’s content. Search engines often use it for the snippet in search results. A meta description tag should generally inform and interest users with a short, relevant summary of what a particular page is about. They are like a pitch that convinces the user that the page is exactly what they’re looking for.
The meta robots tag tells search engines if and how they should crawl your web pages. Use meta robots tags only when you want to restrict the way Google crawls a page, and don’t block pages with meta robots tags in robots.txt.
A meta viewport tag sets the visible area of a web page. It is used to instruct the browser how to render the page on different screen sizes (i.e., desktop/tablet/mobile). Use of this tag indicates to Google that the page is mobile-friendly, which is significant because Google ranks mobile-friendly web pages higher in mobile search results as of 2015.