A Fredholm operator is like a machine that takes in numbers and gives out other numbers. But this is a special machine because it has some rules that it follows.
Imagine you have a bunch of toy blocks with numbers on them. You put them into the machine, and the machine looks at all the numbers and uses them to create new numbers.
But the machine is picky about what kinds of blocks it can use. It only works with certain blocks, and it has to use all of them to make the new number.
Also, the machine never breaks any blocks. It only uses the blocks to make new numbers.
Another thing is that the machine doesn't always give you a new number. Sometimes, it can't create a new number no matter what blocks you give it.
So a Fredholm operator is a special kind of machine that follows all these rules. It's very useful in math, because it helps us understand how numbers can change when we use certain rules.