A Kubernetes platform is a supported version of Kubernetes, the open-source software that decides which server runs each of an app's containers and restarts any that fail, sold with a company behind it. Kubernetes is often shortened to K8s. Red Hat OpenShift and Canonical's Kubernetes are two examples.
First, what is a container? A modern app is split into many small programs, called services. Each service is packed into a container: a package that holds the service's code and everything it needs to run, such as its libraries and settings. A container runs the same way on a laptop, in a data centre or in a cloud. The page on containers and Kubernetes explains the idea in general.
Kubernetes runs containers across a group of machines, called a cluster. Each machine in the cluster is called a node. The engineers write down what they want, for example "run 20 copies of the application service, each with 1 processor and 2 GB of memory". Kubernetes places each copy, called a pod, on a node that has room. It watches the pods all the time. If a pod crashes or a node fails, it starts a new pod elsewhere. When traffic rises, it can add pods automatically, and remove them again when traffic falls.
Consider a state government's scholarship portal in Bihar. On most days, about 5,000 students log in, and the portal runs 20 pods of its application service. On the last date for applications, 4 lakh students try to apply, most of them in the evening. As the load rises, Kubernetes adds pods, up to 120, spread across 30 nodes. At 9 p.m., one node fails. Kubernetes notices within seconds, and starts that node's 4 pods on other nodes. Students whose requests were on that node see one error, and succeed when they try again. The next morning, Kubernetes shrinks the service back to 20 pods.
So why buy a platform, if Kubernetes is free? Plain Kubernetes is a set of parts. A company still has to choose and connect the parts for networking, storage, logins and monitoring. It must also upgrade all of them every few months, because each Kubernetes version is supported for only about a year. A platform seller tests these parts together, provides security fixes, and answers the phone when something breaks. Platforms also add a web console, a place to keep container images, and security rules. The page on the open source business model explains this trade.
New versions of each container arrive from the CI/CD pipeline, the automated system that builds and tests every change, and are stored in an artifact registry. Container image scanning checks each image for known flaws before it runs. The platform sends data about its pods to an observability platform. It runs on an operating system such as enterprise Linux, on virtual machines or directly on servers. A developer cloud also rents out ready-made Kubernetes clusters.