Once you enrolled contacts into a workflow, they proceed through it step by step. In a workflow, contacts are either in one of the steps, or they can be out of the steps.
For every step of the 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:
Contacts wait for the step to be completed.
Example: waiting for the set Delay time, or waiting in the sending queue.
You've scheduled the enrollment for later.
3. 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:
1. Moved to the next step
Contacts completed the step and moved on to the next one.
Contacts for whom the 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 workflow.
Contacts completed the last step of this workflow. They basically received all the steps of the workflow and nothing happened.