My brother was diagnosed with attention deficit hyperactivity disorder. What does this mean?

ADHD. ADHD is common. About 7.8% of kids in the USA have ADHD. If untreated, ADHD may affects lives of children/adolescents significantly. It includes their academic, social and family functioning. The main treatment for ADHD are medications, including psychostimulants and non-stimulant medications.
ADHD. Means that his brain is on overload, so he can't focus on one thing for very long. It also means that his brain is going so fast that his body has to follow.
He has a neuro- developmental disorder with significant symptoms of Inattention &/or Hyperactivity/Impulsivity that harmfully impact his daily functioning at work/school & home in academics, behavior &/or peer interactions. He has an 80% chance of responding to multimodal management of his symptoms by medication, behavioral therapy & environmental manipulations to minimize the functional impairment. See CHADD.org.