Migrating Data to MongoDB Atlas` involves using the Live Import tool to seamlessly pull in an existing MongoDB deployment into MongoDB Atlas. MongoDB Atlas provides features such as security, built-in replication for always-on availability, backups and point-in-time recovery, fine-grained monitoring, automated patching, and a choice of cloud providers, regions, and billing options. To migrate data from an existing database, one needs to prepare in advance by stopping application writes, performing a dry-run of the `mongodump` & `mongorestore` steps, and identifying how the new MongoDB Atlas instance will be connected to the application nodes. The migration process involves creating the MongoDB Atlas cluster, adding IP addresses to the whitelist, backing up data from the existing database, writing data to MongoDB Atlas, switching database connections, and executing the migration. To migrate data out of MongoDB Atlas, one can download a backup, copy the contents to the receiving MongoDB cluster, and load the data into the receiving replica set. To get the best out of MongoDB Atlas, it's essential to consider instance size selection, application patterns, schema design, indexing, and disk I/O when deploying and managing the database.