A synapse is a connection between two neurons: postsynaptic and presynaptic. The first is usually the sender of a "signal" (which maybe taken to be the chemicals it releases or the action potential that triggers this release), while the second is the receiver.
Thus the word presynaptic means something related with the first neural cell, or just that part of a synapse that is in fact a part of this first neuron, whilst postsynaptic means something related to the second cell.
See also: neurotransmitter
