you won't understand it until you've experienced it. sorry.. this i can say for certain.. I don't blame you for not understanding PBL if your whole undergraduate education has been based on a traditional model of learning.
there is literature that documents students' frustrations when first experiencing PBL. It's no secret that it takes a while for students to get acclimatized (hence I understand Dr.Reo's angst and can empathize with him).
I could try to explainwhat I think PBL is, but I don't think it will help you. Instead I will act as a faciltator and urge you to find the resources necessary in answering your question (i'd like to point you to OVID or a similar database). After you do this, you will have formally experienced something close to PBL. Finally, dont forget to come back, and talk with us about what you did.
So.... proceed my young sappling...