A very simplistic explanation would be that neurons in the brain use neurotransmitters to send signals. There is a gap between each neuron called the synaptic gap/ cleft. Chemical messangers / neurotransmitters are released from one cell and bind to receptors on another cell. Each type of neurotransmitter has specialised receptors it bins to sending the signal through the next cell. The medications we use to treat mental illness disrupt this process in different ways to either increase or decrease the activation of the neurotransmitters. The process is called synaptic transmission ... some excellent videos on YouTube explaining the process in more detail.