Panda B is a fictional street drug named by a format of the likes of "Special K" and such so that it may resemble the name of a real drug. It was likely formulated as a result of public school officials needing a "control question" in order to determine if students were being sincere in their answer choice by asking if they had done a fictional drug in the past month.
Since then, it has been typically used to refer to various non-drugs that are dealt among students, such as candy or other quantities of consumable things. It is also used as an excuse for acting high or drunk.
Guy 1: Man I feel like shit today...
Guy 2: Lay off the Panda B, brah