I remember explaining this to a friend when I did my DNA test and she was asking questions. Here is how I told it to her.

Lets say DNA is like socks. Let's say my mom had 50 black socks and 50 white socks. Well I'm going to inherit only half of her socks. Let's say I'm randomly given half of her socks. I could get only the white socks, only the black socks, 25 of the black, 25 of the white. Hell I would get 40 white and 10 black. It's random. I'm getting half but I don't know what half.