What It Is
CSS, short for Cascading Style Sheets, is the styling layer of a website. HTML says "this is a heading" and CSS says "make headings dark blue, in this font, with this much space below." Going back to the house analogy, if HTML is the framing, CSS is the paint, flooring, trim and furniture arrangement. One CSS file can style every page on a site at once, which keeps the look consistent.
Why It Matters for Your Business
CSS is what makes your site feel like your brand. It applies your colors and fonts everywhere and controls how the layout rearranges on a phone versus a laptop. Because styles are shared, a change like updating your brand color can happen in one place instead of on fifty pages. Well-written CSS also loads quickly, which matters for both visitors and search rankings.
Good to Know
Many WordPress themes have a box for "Additional CSS" where you can paste small style tweaks. It is handy, but it is easy to pile up conflicting fixes over the years. Keep a short note of every change you add and why. If the list grows long, it is usually a sign the theme no longer fits and a cleaner design is worth considering.