One of the eligibility criteria for the PMP exam is to have minimum 3 years / 36 months unique non-overlapping professional project management experience during which at least 4,500 hours were spent leading and directing project tasks.
So, what does "non-overlapping" experience mean?
To answer this question, let me show the reverse situation - overlapping experience. Let's say you have managed 2 project in 2009. Project 1 ran from Jan'09 to Jun'09 (6 months), and Project 2 went from Apr'09 to Dec'09 (9 months). In this case, you have an overlap between the two projects during Apr'09 to Jun'09 (3 months). So, the experience from these 3 months is counted as 3 months, and not 3 x 2 = 6 months. So, your total unique non-overlapping experience is 12 months, not 15 (6 + 9) months.
The PMP Handbook has a very nice example with a diagram on Page 7, to explain this requirement.