ADHD is very easy to describe: It's a condition when your head is either completely empty from any thought or it is so crammed full of thoughts you can't hear yourself thinking. It makes you want to procrastinate for days or it compels you to do everything at the same time, or perhaps you just want to do one specific thing but you can't remember what it was. It's a condition that makes you hungry all day long but you forget to eat all day because you're so hyperfocused on your latest hyperfixation.
You Will know if you met someone with ADHD because they Will either won't stop talking about one very specific topic, or they Will just go from one topic to the next without stopping. Or they won't talk at all. For hours. They Will just space out while staring into the distance, lost in their own thoughts.