A synapse is the junction between the two neurons. Consider this as a joint in an electric wire junction. In an electrical wire, electricity passes two wire through the joint by electron transfer. Here impulse pass from one neuron to another through chemical substances, which we call Neurotransmitter.
A human brain contains millions of neurons in the brain and billions of information pass from one neuron to another. This transmission becomes possible due to the presence of synapse, the gap between the neuron. This gap is the extracellular space where axon secretes neurotransmitters like Acetylcholine, serotonin etc. Neurotransmitters travel to the dendrite of next neuron and spread the impulse to several dendrites.