Once you enrolled contacts in a campaign or a workflow, they proceed through it step by step.
For every step of the campaign/workflow, you can see the number of contacts in and out, and the details about their status.
There are 3 statuses that a contact can have in a step:
Waiting. Contacts wait for the step to be completed.
Example: waiting for the set Delay time, or waiting in the sending queue.
Scheduled. You've scheduled the enrollment for later.
In Error. The step could not be processed due to an error.
You may find more info on how to understand why your emails are not being sent here.
A contact can be out of a step for 3 different reasons:
Moved to the next step. Contacts completed the step and moved on to the next one.
Exited. Contacts for whom the campaign/workflow stopped on this step because they triggered an exit rule (more about the Disenrollment Strategy here) or a user manually removed them from the campaign/workflow.
Completed. Contacts went through all the steps and completed the last step of this campaign/workflow.
If the contact has the status exited or completed, they are no longer in your campaign/workflow. Any other status means that the contact is still in the campaign/workflow.