Docker is a container platform used to package an application, its libraries, and runtime into an isolated unit called a container. Containers share the host kernel, so they are lighter than full virtual machines, but they are not a security boundary by themselves.
In cyber security, Docker matters because many internet-facing services run in containers. If an attacker finds a web flaw inside the app, they may try to abuse container settings, exposed Docker sockets, weak permissions, or vulnerable images to reach the host system. Defenders use Docker to reduce environment drift, deploy patches quickly, and limit damage with non-root users, read-only filesystems, and tight network and volume controls. Good container hygiene helps prevent a small app compromise from becoming full-system control.