Deployment is the process of putting an app or website into an environment where people can use it. It is like moving a finished project from your workbench into a public shop.
For a website, deployment may copy the tested files to a hosting service and make them available at its web address. A team might first deploy to a private test site, then make the same changes available to everyone.
Deployment does not guarantee that everything works perfectly. Teams usually check the live result and keep a way to undo a change if it causes trouble.