On December 26th, 2017, the largest prime number ever discovered was found by Jonathan Pace of Germantown, Tennessee. The number is over 23 million digits long, so it’s easier to call it by its nickname, M77232917. It gets this name because that’s the exponent you raise 2 by to find it. Once you’re done multiplying 77,232,917 twos together, subtract 1 and just like that, you have the largest prime number known to date.
Prime numbers found by raising two by a prime exponent and subtracting one are called Mersenne primes. They’re named for a 17th century French friar, hence the capital “M” in the shorthand name. Three is a Mersenne prime, since it’s two to the second power minus one. Same for 7, which is two to the third power minus one.