CSS, Cascading Style Sheets, is the style sheet language that controls how HTML elements actually look: color, spacing, size, typography, layout, and positioning. HTML defines what's on the page and how it's structured; CSS defines how that structure gets presented visually, the division of labor that lets the same HTML render completely differently depending on the stylesheet applied to it.
For SEO, CSS mostly matters indirectly, through performance and rendering rather than through content itself. A bloated stylesheet, one loading far more rules than a page actually uses, adds to page weight and can block rendering while the browser waits for it to load and parse. CSS is also how content sometimes gets hidden from view (setting display:none or moving text off-screen), which search engines can detect and, depending on the intent, treat as a normal design choice or as an attempt to hide manipulative content like keyword stuffing from visitors while still serving it to crawlers.
