What does the "3,4" mean in 3,4-methylenedioxymethamphetamine (MDMA)?
Numbers in organic molecules tell you where so-called functional groups (important configurations of elements) are attached to a central structure.  The central structure in MDMA is a benzene ring $\ce{(C6H6)}$, with the main functional group attached at a carbon which makes methamphetamine.  This carbon is labeled 1 as the "most important" functional group.  To get to MDMA from methamphetamine, you add a methelynedioxyl group, and this attaches at carbons 3 and 4.  So that's where those numbers come from!

EDIT: Below is a molecule of MDMA:



The hexagon shape near the center is the benzene ring.  The group to the right, attached to the benzene ring at about 2 o'clock is the main functional group that turns benzene into methamphetamine.  That position on the ring is labeled 1.  Now all other functional groups are numbered sequentially in the direction that makes the next functional group nearest to 1.  So that means we go counterclockwise, and the group on the far left with the 2 O's (oxygens) is attached at positions 3 and 4.