I really like some of the ways people have already posted!
When I found out that not only have anxiety and depression, I also have bipolar type my mom instantly read everything there was possibly to read on it. She asked my permission to meet with my counselor on her own so she could asked questions and find out even more information.
What I normally say is it is almost like you are experiencing the "flight or fight" reaction but there is nothing to fight or flight to. For bipolar most people already can imagine the ups and downs, the only thing I add is one day everything is good, you wake up and you are so depressed you cant get out of bed and vice versa. If people want to know more than that, I tell them that there is a lot of information they can look up and read on google.