A CDN (content delivery network) is a network of servers in many locations that stores copies of a site's files and serves them from the point nearest the user. The result: the site loads fast no matter which country the visitor is in.
Without a CDN every visitor downloads the site from one server – say, in Warsaw; a user in Lisbon waits longer because the data physically travels further. A CDN keeps copies of the files in dozens of locations and serves them from the nearest one, cutting load time and improving Core Web Vitals.
The second gain is resilience: a CDN absorbs sudden traffic spikes (a campaign, a media mention) and filters some attacks, including DDoS. In the JAMstack architecture the whole site lives on the CDN – which is why such sites survive loads that would take down classic hosting.
Practical note: overly aggressive anti-bot settings on a CDN can block search-engine crawlers and AI bots without the owner's knowledge – quietly cutting the site out of Google and out of ChatGPT's answers. After enabling protection, check the logs to see who is actually getting a 403.