Matching law is when we try to divide our time and attention equally between two or more things. For example, imagine mommy or daddy gives you two different types of candy to enjoy. Maybe one is a lollipop and the other is a chocolate bar. You might try to give equal attention to both candies so that you can enjoy them both fully. This is what the matching law is like - trying to give equal attention to different things so that we can enjoy them all to the fullest.
But sometimes, we might like one candy more than the other. Maybe we really, really love the lollipop and only kind of like the chocolate bar. If that's the case, we might spend more time eating the lollipop because it's so yummy. The matching law says that we tend to spend more time on things we like more than things we like less.
This isn't just true for candy, though - it's true for lots of things. For example, if you have two different toys to play with, you might prefer one over the other and spend more time playing with it. Or if you have two different TV shows to watch, you might prefer one over the other and spend more time watching it. That's the matching law in action!