Replication is a form of horizontal scaling where your project is deployed on multiple copies of the same infrastructure simultaneously. Web traffic or other server load is spread between the replicas to improve your project's speed and reliability.
To enable replication on a project, use the "Resources and Replication" pane in the General Settings tab of the project's dashboard.
Replicas are expensive; they are a multiplicative force on a project's cost. To ensure you only pay for replication when it is necessary, consider autoscaling, which will automatically spin up and down replicas as needed to handle load.