The title tag is the <title> element in the code of every page, and it is also the text on your browser tab. Google calls the clickable title of a search result a title link, and Google creates it automatically for each result.
According to Google’s explanation of title links, Google draws on sources including the title tag, the main heading of the page, og:title, other prominent text, link text pointing to the page and WebSite structured data.
How do you write a good title?
Google gives these guidelines: give every page a title, make it descriptive and concise, avoid vague titles such as “Home”, do not repeat keywords and avoid boilerplate text that appears on every page. If you want your brand name in it, put it at the start or end, separated by a character such as a vertical bar. There is no maximum length, but Google truncates title links where needed, usually based on screen width.
Example
<head>
<title>Dormer windows fitted in Apeldoorn | Hendriks Builders</title>
</head>Common mistake
Using the same title everywhere, such as the business name. Searchers then cannot see how pages differ, and Google is more likely to choose other text as the title link. Make the main heading (H1) and the title tag cover the same thing; Google writes that the main title should clearly be the most prominent text on the page.
See also meta description and the Technical SEO checklist.