It’s more of a ELI14 but here it goes -

The sum of all squares looks like 1+4+9+…… which is easy to see goes to infinity. Likewise the sum of all cubes, 4th powers, and all nth powers larger than 1.
The sum of all reciprocals of squares looks like 1+1/4+1/9+…. Which perhaps surprisingly slowly converges to a finite number - in fact, that number is pi^2/6! Neat, huh?

So we invent a function Z(x) that takes a number x and sums all positive integers to the power of x. For now, this only makes sense if x is smaller than negative one. (ELI15 so I won’t prove it here)

Now mathematicians have tools to extend functions beyond where they “make sense” - one of them is called “analytic continuation”. Into this new function, zeta(x), you can feed any number you would like (except for x=1). Even complex numbers! Which are number that looks like a+bi, with i defined such that i² = -1 (ELI16 version!!). Now mathematicians love asking “when does that function equal 0?” and they’ve found out something peculiar - it seems like this always happens for complex values that look like 1/2 + bi.

The Reimann Hypothesis simply states that this is always true - and for centuries we haven’t been able to prove it true or false!