For the OP.
Simply stated, it's like a production line. You start with one (or more) plant (or plants). Every week you add one (or more) new ones to the "line". It is mandatory to keep track of each plants' "place in line" After 4 weeks (or whatever) of vegging, you move the first plant (or plants) into flower mode. From this point on; each week you will add one (or more) new plants into veg - as well as transfering one (or more) plants into flowering mode. So, in effect, each plant (or batch of plants) moves up "one place in line" each week.
So after about 12-15 weeks, or so, the oldest plant (or plants) should be ready to harvest. When you harvest the oldest batch, you put a new batch in the other end - and just keep doing that every week.
That's how it works! You have 12-15 (or so) single plants (or batches of plants) in "graduated" stages of developement - ranging from the smallest babies up to the ones that are ready to harvest. Each week you move them all up one space in line and that's what we call a "Perpetual Harvest".