When executing a deployment process with multiple steps, Octopus runs all of the steps sequentially; it waits for the first step to finish before starting the second, and so on. However, sometimes this isn't desired. If you are deploying to a farm of 10 web servers, it might be nice to deploy to one machine at a time, or to batches of machines at a time. This is called a rolling deployment.
Octopus 2.0 now supports rolling deployments with child steps - learn more in the documentation on rolling deployments.
You can also see a five minute video on rolling deployments in action.