Rapid Development Environment (RDE) in AEM as a Cloud Service is a new environment that you could include in your dev or prod instance on demand for quick deployment, this helps the AEM developers to quickly deploy the changes from local machine build just like we are doing package and bundle install using package manager.
Since AEM as CS package manage doesn't allow to deploy the code changes due to immutability of the system, but AEM as CS package manager allow to upload and install the content packages that includes paths like /content, /etc.
Here, instead of using package manager for deployment RDE is using AppBuilder and pushing the locally build zip, jar files in AEM as CS RDE instance. Below are the detail about the Rapid Development Environment in AEM as CS development environment setup and uses of it.
How to setup Rapid Development Environment in AEM as CS?
1. First we need to enable the RDE instance for that you have to contact Adobe CSE/Support to get that enable under your license.
2. We need to install node and adobe cli to make the development setup and running in local machine.