Distributed Denial of Service - attack that overwhelms a service with traffic from many sources. Like having thousands of people simultaneously calling a restaurant to prevent real customers from getting through.
Online services use DDoS protection to stay available when attackers try to overwhelm their servers with fake traffic.
All four providers offer managed DDoS defense, but they’re packaged differently: AWS Shield and Azure DDoS Protection are dedicated DDoS services; GCP primarily delivers DDoS mitigation through its global edge plus Cloud Armor (WAF/rate limiting) and related networking features; OCI provides DDoS mitigation as part of its edge/network protections. In practice, you combine DDoS protection with CDN/edge, load balancing, WAF, and rate limiting.