Kubernetes is an open source container orchestrator. Google initiated this project in 2014, inspired by Borg (see original article, the original paper, and second paper). It has turned into one of the most successful and active open source communities, eclipsing earlier similar projects such as Apache Mesos or Docker Swarm et al. Google is the company with most contributions to this open source community.
The Cloud Native Computing Foundation (CNCF) is the steward of an ecosystem of 50+ projects (and sandbox projects) that has spawned up around Kubernetes, with 97k people from 177 countries contributing 1.3B lines of code, as of September 2020 (and as of August 2019, 35k contributors). There are many more Kubernetes related open source projects not hosted by the CNCF. There is also a vibrant ecosystem of vendors offering cloud native Kubernetes related commercial products, solutions and services.
Wikipedia has Kubernetes' History, or e.g. this Origin Story about Kubernetes (AKA k8s).