The title tag is the text inside the page's title element. Search engines often use it as the blue link in results, though they may rewrite it if it does not suit the query. It also appears in browser tabs and when a page is shared or bookmarked.
Write one that is unique to the page, describes it accurately and puts the important words near the start. Keep it short enough that it is not cut off, which depends on pixel width rather than a fixed character count, and avoid stuffing it with keywords. A brand name at the end is fine.
It is separate from the on-page H1, and the two can differ, though they should clearly describe the same topic. Pair it with a helpful meta description, because the two together shape whether someone clicks.
Related: Search Engine Optimization, Search Intent, Technical SEO.