Task types that send work to partners (Collaboration - simple, Collaboration - full and Send) require preparation to instruct the recipient about the task. It can also be useful to indicate that the preparation is complete, prior to sending the task.
The Set ready option is available when the task is being prepared. This changes the task status to ready and removes options to change the task definition. It will also check if the task has been assigned and if necessary the status and next steps will indicate that the task needs to be assigned.
The Prepare option is only available when the task status is ready. This requires confirmation. It changes the status back to prepare and makes the options to change the task definition available again.
The set ready and prepare options are available from the task context menu, and in the Task section of the Task details page.