Adhd is complex, OP. It probably isn't possible to explain it in a short post on MN.
It isn't quite as simple as being easily distracted and struggling to concentrate, though that is certainly a part of it. Sometimes, people with adhd can hyperfocus on certain tasks, and it can then be very difficult for them to break away and do something else. This might be why your dd believes that people with adhd can concentrate well sometimes...we can, we just can't switch it on and off at will.
Then there is the other stuff. The brain racing at a billion miles an hour and never stopping...it's utterly exhausting. Like little explosions going off from every angle and you feel compelled to turn round and look at each one. Some of us have a lot of nervous energy that fiddle toys can help with...imagine having ants in your pants all the time and never feeling calm or still, even when you're trying to go to sleep at night.
Then there is the task paralysis... the complete inability to get started on certain tasks, even really simple ones. I will sometimes sit and freeze rather than getting up and putting a cardigan on, or delay going to the toilet until I'm really desperate, because the effort that would be required just seems completely overwhelming.
ADHD impacts on so many aspects of my life, and it's awful. I mask really well most of the time, though, so most people have no idea that I even have it. I have a good job, a functional family, I volunteer in the community etc. Only those closest to me really know how incredibly negatively it affects me.