Cilium is an open-source software solution designed to provide networking, security, and observability capabilities for containerized applications and microservices. It leverages eBPF (extended Berkeley Packet Filter) technology within the Linux kernel to offer high-performance and scalable network connectivity, with features including load balancing, network policy enforcement, and application-level security. Cilium is particularly well-suited for cloud-native environments and is often used with Kubernetes to enhance networking and security functions.