Heroku and MongoDB Atlas are integrated to provide a secure cloud-based app development and deployment environment, ideal for modern applications. MongoDB Atlas offers industry-grade security controls such as encrypted data, authentication, and IP whitelisting, while Heroku Private Spaces offer enterprise-grade secure network topologies. By peering a Heroku Private Space with a MongoDB Atlas cluster running in AWS, companies can securely access their data from fixed IPs or CIDR blocks, ensuring that only approved applications can access the database. The tutorial outlines the steps to initiate and approve the VPC Peering request, verify its activation, and run an app connected to the MongoDB Atlas cluster, providing a secure integration between Heroku and MongoDB Atlas for cloud-based app development and deployment.