CSS is Cascading Style Sheets, the language used to control the visual presentation of HTML content.
It matters because development decisions affect speed, security, scalability, tracking, SEO, and maintenance. A site that looks good but is fragile or hard to update can quietly hold back every marketing channel.
Example
A card layout displays three columns on a desktop and one on a phone. CSS sets those breakpoints, spacing, colors, and type sizes while the underlying HTML content remains the same.