Short answer: DDoS protection is network filtering that absorbs denial-of-service floods before they reach your server. Without it, a host usually null-routes an attacked IP to protect other customers, which takes your server offline until the attack ends.
DDoS protection explained
Distributed denial-of-service attacks flood a server with traffic from many sources. Volumetric attacks (UDP floods, amplification) are measured in gigabits or terabits per second; protocol and application-layer attacks target TCP handshakes or HTTP.
Providers with protection route traffic through scrubbing systems that drop attack packets and pass legitimate traffic. Coverage varies from always-on basic filtering included in every plan to paid, game-aware mitigation with higher capacity. Game servers, voice servers, VPNs and public APIs are the usual targets.
See it in the data
Frequently asked questions
Do I need DDoS protection?
For game servers, TeamSpeak, public proxies and anything that attracts attacks, yes. For a small website behind a CDN, the CDN often provides it.