ADHD may present with many brain-network anomalies, but I believe the classic case is one where there is more default-mode activity, less executive control activity, and ineffective switching occurring from the salience network. Mindfulness meditation is honed at training the salience and attention networks towards playing closer attention, which offsets the deficits observed in ADHD.
That's my lay interpretation; but actually, I believe that people who suffer from ADHD probably have even more to gain, relatively, than those who don't from taking up the habit of mindfulness meditation. It's not an easy fix--I've read that it takes about twice as long for those with ADHD to benefit from the practice. But it seems like it's worth it; after all, your mind is really the only tool that you have.
Those interested in this topic should read about ADHD and it's relation to the salience and executive networks; and how mindfulness sharpens the function in these areas.