containerd
In plain terms
A lightweight, industry-standard container runtime.
Going deeper
containerd manages the container lifecycle, pulling images and running containers, used under Docker and Kubernetes. It's a core, low-level runtime.
Under the hood
A CNCF project; Kubernetes uses it directly via CRI, having dropped the Docker shim.
Examples
Kubernetes runtime, Docker's core, CRI, low-level container management.
Think of it like...
Like the engine under the hood that actually runs the containers.