Excitatory neurotransmitters depolarize the postsynaptic neuron by making the inside of it more positive and bringing it closer to its action potential threshold, making it more likely to fire that message on to the next neuron.
Since it's thought that low levels of neurotransmitters are involved in ADHD, stimulants are used because they tend to increase the number of neurotransmitters, like dopamine, between neurons, therefore reducing symptoms.
Some neurotransmitters bind to the receptors and tell the cell to open up the ion channels and relay an electrical message, and these are called excitatory neurotransmitters.
Some researchers suspect that this deficiency takes the breaks off glutamates excitery activity thus stimulating things like repetitive behavior documents money and then his colleagues produced evidence supporting this idea.