CDN stands for content delivery network. It is a set of computers in data centres all over the world, each holding a copy of your website. When somebody opens your site, they are served from the nearest one instead of from a single machine that might be anywhere.
What it actually changes
Distance costs time. A request from Wakefield to a server in Manchester is quick. The same request to a server in Texas has to cross the Atlantic and come back, which adds a noticeable delay before anything appears.
A CDN removes most of that by keeping a copy close to the visitor. For a UK trade site the practical gain is on mobile, where the connection is already the slow part and every saved round trip shows.
The other things it does
A CDN absorbs traffic spikes, so a page that gets shared in a big local Facebook group does not fall over. It handles the HTTPS padlock. Most of them block obvious junk traffic and bots before it ever reaches your site.
The main providers are Cloudflare, Fastly and Amazon CloudFront. For a small business site, a free Cloudflare plan covers everything you need, which is why most sites we build sit on one.
A worked example
A drainage firm covering Oklahoma has a site hosted in the UK because that is where the designer was. Every American visitor waits for a transatlantic round trip before the first pixel appears.
Put the same site behind a CDN and the American visitor is served from Dallas, the UK office still sees it served from London, and nobody changed a line of the site.