Simplified: Instead of modifying a deployment by uninstalling the old software, updating the config files and installing a new version, you create a new virtual

Simplified: Instead of modifying a deployment by uninstalling the old software, updating the config files and installing a new version, you create a new virtual machine image with all you need and deploy instances of that. Even if you just want to change a setting in a file you deploy new virtual machines. There are variations of this such as using Docker.