What is the meaning of extension in html

Question

I was give that the extension of HTML is .html or .htm and why it need to be added in any Html file

Answer 1

audio file saved as audioname.mp3

Video file saved as videoname.mp4 or
video name.ogg

Similarly html file is saved as
filename.html or filename.htm

to ease the browser to recognise the filetype of file.

Here .mp3
.mp4
.off
.html
.htm
these are called extension.

Answer 2

So the browser knows, that it is a markup file that it can translate into graphical webpage by reading the HTML inside.