A title tag is the page title set in its HTML. It's shown in browser tabs and is usually the clickable headline of the page in search results.
Search engines show roughly the first 50–60 characters, so put the main words first.
Each page should have its own title that says what that page is about, not the same site name everywhere.
Why it matters for a launch
The title is one of the strongest signals of what a page is about and the main thing a searcher reads before deciding to click.