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 provide managed defenses against DDoS attacks. AWS Shield and Azure DDoS Protection are dedicated DDoS services; GCP commonly uses Cloud Armor (WAF + DDoS mitigation) in front of HTTP(S) load balancers; OCI DDoS Protection provides network-layer DDoS mitigation for OCI services.