One way that we can answer this is to break down the three parts that most websites are usually composed of: structure, styling and interaction.
HTML is the structure of a website. You can then use further languages to layer on styling and interactivity. We can help to reinforce this concept by thinking of a building, the HTML would be the hardware that gives it the structure, such as the walls, roof etc. A website can contain a multitude of types of content, such as text, images and videos. The HTML structures this within the page.
You can then use a language (such as CSS) to add styling. Thinking about our building analogy, the styling would be anything that we could add visually to the building, such as the colour of the walls, the colour of the doors etc.
We can then use a language (such as JavaScript) to add interaction. With our building analogy, the ineraction would be adding a door bell, a light switch etc.
HTML is an acronym and stands for HyperText Markup Language:
HyperText is text that provides access to other text through links, known as hyperlinks.
A markup language is a computer language that defines the structure and presentation of raw text.