The term "stop" refers to a type of step that can be incorporated into your automation. The stop step collects contacts at the end of a workflow branch.
Using a stop step allows you to add more to the workflow in the future. Build out the rest of the branch after the Stop, when you're ready to move those contacts forward, remove the Stop. They'll flow to the next step.
Simply drag the Stop element to the desired section of your workflow. No further editing is needed.
If you plan to use the automation re-entry setting, Stop steps should not be used at each end of a branch.