A neurotransmitter is a chemical that nerve cells release to pass signals to other nerve cells, muscles or glands, such as dopamine, noradrenaline and serotonin.
One nerve cell releases it into a tiny gap. It attaches to receptors on the next cell, which changes that cell's activity. It is then broken down or taken back into the first cell, which is called reuptake.
Dopamine and noradrenaline are the two most linked with ADHD. They are involved in attention, motivation and how rewarding a task feels. Stimulant medication increases how much of them is available between nerve cells, mainly by slowing reuptake or increasing release.
You will read that ADHD is a 'dopamine deficiency'. That is a simplification. The research suggests differences in how dopamine signalling works rather than a simple shortage.