Imagine you are playing with some building blocks, but you can only use the shapes that perfectly fit together. Let's say you have some square blocks and some triangle blocks. The square blocks can only fit with other square blocks and the triangle blocks can only fit with other triangle blocks.
In math, we have something called differential forms that are like building blocks for equations. Some forms are very picky about which other forms they can fit with, just like the square and triangle blocks.
A closed differential form is like a square block - it can only fit with other closed forms. A closed form is one where if you drew a loop around any point, the amount of the form inside the loop would always add up to zero. This is kind of like saying if you have a cup of water and you pour some water in, then you scoop some out, the amount of water left in the cup is the same as when you started.
An exact differential form is like a triangle block - it can only fit with other exact forms. An exact form is one that is the derivative of another form. This is kind of like saying if you have a big cake, and you take a slice out of it, you know exactly how much cake is left over.
Some forms are both closed and exact, like a square with a triangle on top. These forms are like puzzle pieces that can fit with both closed and exact forms.
In math, we use differential forms to help us solve big problems. Just like building blocks, we can put different forms together to make more complicated equations that help us understand things like the shape of the Earth or how electricity flows through circuits.