The equinox is like a special day when the day and night are almost the same length. Imagine the Earth as a big ball that spins around. Sometimes, one half of the ball gets more sunlight than the other half. But on the equinox, the sunlight shines right at the middle, making both halves get the same amount of light. It happens twice a year, in spring and fall, marking the change of seasons.
Think of the equinox as a balance game. If you have a see-saw and you put the same weight on both sides, it stays perfectly balanced. On the equinox, the day and night are balanced just like that see-saw, neither too long nor too short.
Another way to think about it is like a birthday for the seasons. Just like you have a birthday to celebrate getting older, the equinox is a celebration for the Earth, when it gets ready to change its clothes from winter to spring or from summer to fall. It’s a special time when everything feels just right, not too hot and not too cold.