P1, P2 and P3 refer to Period1, Period2 and Period3.
In a real situation, they could be labelled week1, week2, week3,.....weekN
so if you have data collected for 2 years (i.e. 52 * 2 = 104 weeks) you could create a report comparing data across those 104 periods.
Normally I do for the last 10 periods.
hope I answered your question. If still not clear, please bug me. I will try to be more explanatory.