The Riemann hypothesis is one of the Clay Mathematics Institute's Millennium Prize Problems. Here is the problem, as written by the CMI: "The prime number theorem determines the average distribution of the primes. The Riemann hypothesis tells us about the deviation from the average. Formulated in Riemann's 1859 paper, it asserts that all the 'non-obvious' zeros of the zeta function are complex numbers with real part 1/2."
Solve this problem (or any of the Millennium Prize Problems), and you'll be awarded $1 million. Because this problem remains unsolved, surely you've guessed that it's pretty complicated to explain in simpler terms. It deals with prime numbers and functions, and we'll let the video below parse out the rest.