When electrical impulses are fired around your body, they do so by travelling around in neurones as chemical/electrical impulses.
However, your neurones don't cover every inch of your body. To get over this issue, there are small gaps between the neurones - this is called the synapse. Technically speaking, it's a junction between the neurone and the next cell. , the gap itself is called the synaptic cleft. The neurone before the gap is given the name of a presynaptic neurone - it has a swelling on it called the synaptic knob. On the presynaptic neurone there's synaptic vesicles filled with chemicals called neurotransmitters.
When an action potential reaches the end of the neurone, neurotransmitters are released into the synaptic cleft. They diffuse across this gap and attach to the postsynaptic membrane - here they bind to specific receptors. When the neurotransmitter binds to the receptors, this could trigger an action potential, causing muscle contraction (perhaps) in the next cell. Alternatively, a hormone could be secreted if it's a gland cell.
Because these receptors are only on the post synaptic membrane, this makes action potentials unidirectional - i.e they travel in only one direction. If nerve signals travelled in multiple directions, things would get very confusing very quickly!
