Rules for how a page looks
HTML identifies parts of a webpage, such as headings, paragraphs, and buttons. CSS tells the browser how to show them: their colours, sizes, spacing, and layout. A set of CSS rules is a stylesheet; one rule can style every heading on a site.
When two CSS rules disagree, the browser uses a set of priorities to choose which one wins. That is the “cascading” in CSS.
Looks affect usability
CSS can rearrange a page for different screen sizes and highlight the button or link a keyboard user has reached. Text size, contrast, and spacing also affect how easy a page is to read and use.