Definition
A DDoS attack attempts to make an online service unavailable by overwhelming it with traffic. The 'distributed' part is what makes it hard to defend against: instead of a single attacker, the traffic comes from thousands or millions of compromised devices — a botnet — making it impossible to simply block the source. Attacks are measured in volumetric terms (gigabits per second of junk traffic), in packet rate (millions of packets per second), or by application-layer effects (exhausting database connections, memory, or CPU).
Why a VPN Helps Individuals
Online gamers, streamers, and high-profile activists are sometimes targeted personally. An attacker who learns the victim's home IP can flood the residential connection until the ISP drops it. A VPN hides the home IP behind the IP of the VPN server, which sits on infrastructure designed to absorb and filter junk traffic. The attacker, even if they know your username or handle, cannot reach your home network directly because the VPN provider has thousands of times more capacity than your residential link.
VPNs Cannot Protect Servers
If you run a public web server, a VPN does not help — the server's IP must be reachable for users to find it. Server-side DDoS protection requires a different layer: services like Cloudflare, AWS Shield, and Akamai sit in front of the origin server, scrub malicious traffic, and pass only legitimate requests through. The two defenses serve different purposes.