Okay, so imagine you have a dog. The dog is walking near a big puddle of water. Now, when your dog looks at the water, it doesn't see another dog like you would when you look at it. Instead, it sees its own reflection.
A reflection is like a picture of your dog, but in the water. It's a bit like a mirror - you know when you see yourself in the mirror and it looks like you're there but backwards? That's kind of what it's like for the dog when it sees its reflection.
Now, sometimes dogs get confused when they see their reflection. They might try to play with it, or they might get scared and bark at it because they don't understand what it is. But don't worry, the dog will eventually figure out that it's just looking at itself and not another dog.