CDN stands for content delivery network. Instead of every visitor loading your site from one server in one data centre, a CDN stores copies of your images, code and often whole pages at many locations, and serves each visitor from the nearest one.
That makes pages load faster, particularly images, and it takes the strain off your main server when traffic spikes. Many CDNs also block some kinds of attacks.
Cloudflare, Fastly and Amazon CloudFront are well-known examples. Many modern hosting services include a CDN automatically.
For a local business most visitors are nearby anyway, so the main benefits are speed, reliability and some protection against attacks. It is a sign of decent hosting, and it usually costs you nothing extra when it is built in.