KeystoneJS is an open source framework built on top of MongoDB, Express, and Node.js, which is ideal for building web applications and Content Management Systems. KeystoneJS can be configured to use MongoDB Atlas, a cloud-based MongoDB service that provides flexible schemas, rich queries, high availability, and scaling, as well as security features such as SSL encryption, backups, and fine-grained monitoring. To start using KeystoneJS with MongoDB Atlas, users need to create a user in the MongoDB Atlas UI, download and install Node.js, add the IP address of their application server to the IP Whitelist, update the MONGO_URI value in the .env file, and open port 3000 in their firewall. Once configured, clients can connect to KeystoneJS through port 3000 and test its functionality by signing in and creating new pages and posts. Additionally, users can use MongoDB Compass to visually navigate through the schema and data created by KeystoneJS.