What a title tag actually is
The title tag is the HTML element <title> in the <head> of every page. Google uses it as the primary topical label for the page and displays it (sometimes rewritten) as the clickable headline in search results. Browsers also use it as the tab title and bookmark name.
Note that the title tag is not the same as the H1 — the H1 is the visible heading on the page, and while they often share content, they don't have to be identical. The title tag is the SERP-facing label; the H1 is the on-page label.