CDN
A CDN keeps copies of your site in many cities, so each visitor gets it from a computer that is close to them.
In a simple example
A visitor in Kuwait and one in London both open your site fast, because each gets a copy from a nearby place.
Why it matters to you
Your pages open faster for everyone, and a sudden crowd of visitors does not slow the site down.
Do not confuse with
A CDN does not replace hosting. It sits in front of it and copies what it sends.