Okay kiddo, so imagine you have a lot of friends and you want to invite all of them to your birthday party. But instead of individually writing each friend a separate invitation, you decide to put all their names in one big email and send it to everyone all at once. That's what we call a shotgun email.
It's like shooting a shotgun - you fire a bunch of pellets all at once in the general direction you want them to go. But just like how not all the pellets will hit the exact target you want, not all your friends might actually be interested in coming to your party. That's why some people don't like to use shotgun emails for important things, because it might not be very effective.
Does that make sense, kiddo?