Jenkins X is a CI/CD solution for modern cloud applications on Kubernetes and is being proposed as a sub-project via JEP-400.
Jenkins X has the concepts of Apps and Environments built in. You do not directly install Jenkins to use Jenkins X, Jenkins is embedded as a pipeline engine as part of the installation for a team. Jenkins X does not aim to replace Jenkins, but builds on it with best of breed open source tools.
Think of it as a great way to achieve CI/CD with batteries included, without having to assemble things together yourself.
Rather than having to have deep knowledge of the internals of Jenkins Pipeline, Jenkins X will default great pipelines for your projects that implements full CI and CD.
Each team gets a set of Environments. Jenkins X then automates the management of the Environments and the Promotion of new versions of Applications between Environments via GitOps
Jenkins X automatically spins up Preview Environments for your Pull Requests so you can get fast feedback before changes are merged to master
Jenkins X automatically comments on your Commits, Issues and Pull Requests with feedback as code is ready to be previewed, is promoted to environments or if Pull Requests are generated automatically to upgrade versions.