This animation shows a network of neurons and a signal (aka action potential).   In general, when a neuron receives enough input,  it fires an action potential that travels  down its axon.  A single axon may form many branches and include thousands of  terminals (output points).   These terminals (aka boutons) maintain pools of vesicles that contain neurotransmitters such as glutamate.    Near the end of the animation, the action potential arrives at a terminal.   Although not shown explicitly,   the action potential causes voltage gated channels to open such that calcium  rushes into the terminal and binds to calcium sensors.   This causes some presynaptic vesicles to fuse with the membrane and release their contents into the synaptic cleft (a ~20 nm gap between the output  and input terminals).   The released neurotransmitters bind to receptors (ligand gated) on the postsynaptic neuron.   These receptor channels open and allow ions to enter.   So now the signal has been transmitted from one neuron to the next via a "chemical synapse".   Of course this animation omits many details about the strength of the synapse and so on, for example, the probability of vesicle release, the number and locations of the voltage gated channels,  the number, locations, and types of ligand gated receptors, and how the synapse changes over time.