As rice cools down and sits for a while, the starches inside the grains start to Harden and become more separate. This process is called starch retrogradation, and it's what makes day old rice so much better for fried rice. It's like the rice is transforming into a different texture, one that's perfect for stir-frying.
Imagine you're trying to make a beautiful salad, but all the ingredients are stuck together. That's kind of what happens when you try to make fried rice with freshly cooked rice - it all becomes a mushy mess. But with day old rice, the grains are nice and separate, making it easy to stir-fry them with your favorite ingredients.
