What is the function of <title> tag....which is missing here

Answer 1

Title does two things. One, it sets the text in your browser tab. Two, it give a search engine looking at the page a clue to what your page is.

Answer 2

If you look at the tab on your browser it has some text, that is the title tag. It basically gives the user some idea as to where they are on the site. On YouTube, it is used to display the name of the video.