A title tag is the page title displayed in search results, on the browser tab, and when a link is shared. It is written for the human and the search engine at the same time, which is why several practical rules apply: the main keyword belongs as close to the beginning as possible, the length should not exceed roughly sixty characters including the brand name, so the title is not truncated, and each page needs its own title, not a variation on the same sentence. The promise made in the title must match the content of the page, otherwise the visitor leaves immediately and the click-through has no value. The search engine may rewrite the title if it judges it unsuitable, which is usually a signal that it is too long, repeats the keyword, or does not capture the content of the page.
Source: developers.google.com
See also: Click-through rate optimisation, Meta description, Heading hierarchy.
