Imagine you have a bunch of toys, and you have to pick which ones you like the most. Sometimes, you might have a very clear favorite toy that you would choose over any other toy. Other times, you might have a few toys that you like equally, so you don't have a very strong preference for one over the other.
Having monotone preferences means that whenever you have to make a choice between two toys, you will always pick the one that you like more. You will never change your mind and suddenly start liking the toy you didn't choose more than the one you did choose.
For example, let's say your top three favorite toys are a doll, a truck, and a puzzle. If you have to choose between the doll and the truck, because you like the doll more, you will always pick the doll. Even if someone tries to convince you to pick the truck, you won't change your mind because you like the doll more.
Similarly, if you have to choose between the truck and the puzzle, because you like the truck more, you will always pick the truck. Even if someone tries to persuade you to choose the puzzle, your preference for the truck won't change.
The important thing to remember here is that when you have monotone preferences, you always stick with your original choice and you never change your mind.