MJJM13
What Danhiel described above applies to point-to-point tickets only. You’ll get this type of ticket if your destination is located in a different (or none) local fare network than your departure location. Examples for such local fare networks are Passepartout (LU) or ZVV (ZH).
However if you don’t leave your local fare network (or in some cases travel into a partner network) you’ll get a zone ticket instead of a point-to-point ticket. And with such a zone ticket, you may travel as often as you like back and forth within the zones listed during the mentioned validity period.
Hope this explains your confusion.
In your example above, you might have been allowed to ride back to Luzern HB to buy food if you had bought the Luzern City Ticket option offered with your LU-ZH point-to-point ticket. The same applies for using tramways/buses in ZH if buying the additional Zurich City Ticket option.
However for just 1-2 short bus/tramway rides, separate local tickets are often cheaper than the City Ticket surcharge on point-to-point tickets.