While on the Path tab, either in the Master or with the “Edit” button switched on, select a step and then select Settings (the gear icon in the upper right corner).
One of the options here is the "Require completion" toggle button.
If you toggle this button on (the default), learners will need to complete the step in order to finish the class and receive completion status.
If you toggle the button off, the step will be marked as "optional." Learners will be able to skip the step and still receive completion on the class.
Optional steps are not reported in the progress stats. Be sure to keep steps in the default position if you need to track the progress for that step.