Okay, imagine that you have a bunch of toys, and you want to pick one to play with. But you have a bunch of rules you have to follow to make your choice, like that you can only pick a toy that's suitable for your age and that you haven't played with recently.
A decision model is a way of writing down all those rules and conditions so you can figure out which toy to pick.
And notation is the special language we use to write down those rules and conditions so that other people can understand them too. It's like your own secret code that you use to communicate what you want to do.
So, a decision model and notation is just a way of writing down all the rules you need to follow to make a good choice. It's like a roadmap that helps you get where you want to go.