Jays like to hide their food, like acorns and hazelnuts, in many different places. Sometimes, they forget where they put some of their snacks. The nuts that are left behind can start to grow into new trees! This means the forest gets bigger and more beautiful because of the jays' forgotten treasures.
You can also say it like this: 'Jays store nuts in lots of hiding spots, but they don't always remember them all. The nuts they forget can grow into trees, which help make the forest larger and full of life.'