When an action potential reaches an axon terminal voltage-gated calcium channels open and calcium flows inside.
The increased calcium concentration in the axon terminal causes proteins on synaptic vesicles and proteins on the presynaptic membrane to interact. This interaction causes the vesicles and the presynaptic membrane to fuse, releasing neurotransmitter into the synaptic cleft.
Neurotransmitter then crosses the synaptic cleft to bind to neurotransmitter receptors on the postsynaptic membrane of the target cell. An increased frequency of action potentials reaching the axon terminal will usually increase the amount of calcium flowing into the terminal, increasing the amount of neurotransmitter that is released.
When a train of action potentials stops, the voltage-gated calcium channels close, the concentration of calcium in the axon terminal returns to normal, and neurotransmitter stops being released.
This is the way that the information contained in the frequency and duration of trains of action potential is converted into the amount and duration of neurotransmitter released into the synaptic cleft.