A load balancer distributes incoming traffic across multiple servers, preventing any single server from becoming a bottleneck. It sits between clients and backend infrastructure, routing requests based on algorithms (round-robin, least connections, IP hash) or health checks. In practice: your domain points to a load balancer IP, which then farms requests to redundant servers behind it.
Why it matters for bunker domains: load balancers let you run distributed infrastructure without exposing individual server IPs. Useful if you're hosting contentious material, operating across jurisdictions, or want to survive DDoS by distributing load across geographically separated nodes. Also essential for any serious operation — single-server setups fail.
Related concept: reverse proxy does similar work but typically for smaller operations. CDN is load balancing at planetary scale. Bulletproof hosting providers often combine load balancers with DDoS mitigation and multiple upstream links to stay online despite abuse complaints or attacks.