What is HTML and what is CSS?
HTML (HyperText Markup Language) is it authoring language used to write Web Pages.
CSS (Cascading Style Sheet) is used to define styles for Web Pages, including the design, layout, and variations of display for different devices.
Once you have practiced for a while HTML, you will notice quickly that you will need to learn CSS to style it (for example using fonts, colours on the page). Without using CSS your web page will look not really good with the default HTML styling, so CSS is the obligatory next step after HTML.
I prepared a simple image, which can show you how the CSS works on HTML websites:
I hope it will help you to understand more of HTML and CSS and you will like it.