Can you explain what is the CSS?

Cascading Style Sheets, most of the time abbreviated as CSS, is a stylesheet language used to describe the presentation of a document written in HTML or XML (including various XML languages like SVG or XHTML). CSS describes how the structured element must be rendered on screen, on paper, in speech, or on other media.

CSS is one of the core languages of the open web and has a standardized W3C specification. Developed in levels, CSS1 is now obsolete, CSS2.1 a recommendation and CSS3, now split into smaller modules, is progressing on the standard track.Further Reading

HTML has certain defaults for headings and body copy: Without CSS, if you wanted a different but consistent look for these, you would have to specify the appearance at every instance. With CSS, you tell the CSS file how you want headings (for example) to look, then, when a heading appears in your HTML, the presentation engine examines the CSS file to see how it should be formatted.

CSS stands for Cascading Style Sheets:

"It's a stylesheet language used to describe the presention (look and format) of webpage by define display of webpage elements (HTML)".

CSS and HTML are the core technologies for building web pages: HTML provides the Structure of the page while CSS provides the Layout.