When you type "google.com", your browser doesn't know where to go. It needs an IP address like . DNS (Domain Name System) performs this translation.
The lookup follows a hierarchy. Your browser checks its cache, then asks your OS, then your ISP's resolver, then root servers, then TLD servers (.com), then authoritative servers (google.com). Results are cached at each level with TTL (Time To Live).
In system design, DNS matters for load balancing (DNS can return different IPs) and failover (update DNS to point to backup servers).