HTML or hyper text mark up language is a type of text with special characters and codes to 'mark up' or modify text. This  makes a web browser perform an action such as look up and fetch an image or respond to a user input like a click on a modified (marked) piece of text. This action  can change the behaviour of the browser like make it go to another part of a file on an networked computer.

A text file is retrieved by your computer browser in the form of something like this:

<!DOCTYPE html>

    <title>This is a title</title>



    <p>Hello world!</p>



The browser then interprets these pieces of text and then converts (renders) the text file into a combination of images and text laid out in your web browser window. 

Generally the text is 'tagged' with <something> text </something>  that tells the  browser how it should treat the word 'text'.