A shift is a template you assign to a day in the plan. Shifts belong to an employer and are meant for employers in the Shift duty work mode.
When you add an employer, the step Add shifts follows. There you tap the Suggestions Early, Late or Night or create your own templates; you can also Skip it.
Later, open the employer under More → Employer and tap Add shift. The dialog asks for Name, Start Time, End Time and Color. A new employer has to be saved first (“Save the employer first to add shifts.”).
Tap a shift in the employer to change it via Edit shift. When deleting, the app asks “Delete shift” for confirmation.
In the plan, tap a day to assign a shift. Copy whole weeks or months with Copy week or Copy month. More under Entering your plan.