The title tag is a line of text in the hidden head of a page. Search engines show it as the clickable headline of your result, and browsers show it on the tab. It is not the big heading a visitor reads on the page itself, although the two are often similar.
For most searches your title is the first thing a stranger reads about you. A clear one tells them the page answers their question, so more of the people who see you become visitors, and visitors are where leads come from. It also gives search engines one of their strongest hints about the topic of the page.
Write each title by hand, in this order of effort:
Lead with the main phrase a customer would type, such as "emergency plumber in Tacoma", then add your name at the end.
Keep it to about 50 to 60 characters. Search engines cut longer titles off, usually with three dots, and the cut part is lost.
Make it specific. A reader should know what they get: "Kitchen remodeling in Tacoma, free quotes" beats "Our services".
Make it unique. Two pages with the same title compete and confuse readers.
Write for a person. Use plain words and one clear idea.
Most website builders and plugins have a field labelled SEO title or Page title. If you leave it empty, the builder usually falls back to the page name, which is rarely the best wording.
Stuffing in keywords, as in "plumber, plumbing, plumbers Tacoma". It reads as spam and people skip it.
Starting with your brand when nobody knows it yet. Put it last.
Using one title everywhere, often the site name.
Writing titles that promise what the page does not deliver. People leave at once, and that costs you more than a plain title would.
Changing titles on pages that already rank well without a reason. Search engines may also rewrite your title if it does not match the page, so keep it honest.