Riemann hypothesis
The Riemann zeta function is a kind of function. Functions are things in mathematics like equations. Functions take in numbers and give you other numbers back. This is like how you get an answer back when you ask a question. The number you put in is called an "input". The number you get back is called a "value". Every input you put into the Riemann zeta function gives you a special value back. You mostly get a different value for each input. But each input gives you the same value every time you use it. Both the input you give, and the value you get, from the Riemann zeta function are special numbers called complex numbers. A complex number is a number with two parts, a real part and an imaginary part. The imaginary part is called imaginary because you would have to "imagine" such a number as  that when multiplied by itself is equal to . Because based on the rules of arithmetic  and  no such number could exist, it would have to be imagined. Imaginary numbers have vast use in mathematics; without them a lot of technologies would not be possible. As a tangible example, in order to use quadratic formula to solve some equations, the answer sometimes has to be an imaginary number, and historically this was the reason why imaginary numbers were invented in the first place.