When two neurones meet we have a something that acts like a bridge to connect the two neurones together and this is called a synapse. When an impulse is traveling through one neurone and wants to continue along the neurone next to it, it has to cross the bridge (the synapse).
To do this we have neurotransmitters which travel across the synapse and reach the next neurone so that the impulse can travel along that neurone. So at the presynaptic membrane (the membrane of the neurone before the synapse) neurotransmitters are released into the synaptic cleft (which is our bridge- the space between the two neurones), the neurotransmitters diffuse across the synaptic cleft until they reach the postsynaptic membrane (the membrane of the neurone after the synpase). As a result an impulse will be transmitted to the adjacent neurone.