Attention Deficit (Hyperactivity) Disorder is a real phenomenon. It is misunderstood. It is controversial. It is very likely overdiagnosed. But, it exists.
People with ADHD are believed to have lower levels of dopamine and norepinephrine. What this basically means is that high levels of stimulation are needed for an alert state of consciousness (attention). Physically, or mentally, people with ADHD may continually search for interesting information or sensations, which explains an issue with concentration, as the mind perpetually wanders off unless it is deeply engaged. This is problematic, especially in a classic school setting where knowledge is “poured” onto a group of children as if they are one entity with identical learning styles (a different story). You may imagine that a brain with all sensory antennas on overdrive can easily lead to feeling overwhelmed, too. In any case, neurotransmitters such as dopamine and norepinephrine help the brain stay focused, be goal oriented and dismiss information irrelevant to the task at hand. Without the right balance, behaviour can become impulsive and motivated by instant gratification.
ADHD has a bit of an ugly stigma. Outsiders are quick to judge because it may not be an obvious disorder. Adults typically learn to manage their ADHD with professional help, self-development or even self-medication in the form of dopamine rewarding activities (also potentially problematic). Younger children however struggle very much with self-regulation, and as a result can have good and bad days, with no real consistency.
I want to ask for ADHD awareness and consideration. Don’t be quick to glare at the parent of a “naughty” child, and please don’t be quick to assume “bad” behaviour is always due to bad parenting. The child may have frustrated you by not listening, and yelling over the top too many times, but the parent is quite likely feeling worse than you whilst their child has not yet learnt to manage their behaviour. An alternative and welcome approach would be to reach out, send a smile, a suggestion, or even a question.