When we say that a neuron "fires," it means it has reached its threshold for activation and sends out an action potential, which is an electrical impulse, along its axon.
Neurotransmitters are chemical messengers released by neurons that transmit signals across synapses to other neurons or target cells.
Excitatory vs. Inhibitory Signals: Excitatory signals increase the likelihood of a neuron firing, while inhibitory signals decrease the likelihood of firing.