Exponential growth. Each fold doubles the thickness of the paper. If someone gave you 1 penny today, and promised to give you twice as much money tomorrow, and twice as much the day after, and so on and so on, on day 30 they would have given you over $5 million.
You can do the math on this. Open the calculator on your computer, and type in 0.0039 (the thickness of a piece of paper). Press the Times button, and then the number 2, and press enter. Now press enter 102 more times. By the mid-50’s your calculator will actually reach a point where it has to start using exponents, and by 60 you’ll have surpassed the what the calculator is capable of.