“Huh?”, you say. Well, this is how mathematicians (or at least those enamoured of compact and cryptic notation) say
For all positive integers n greater than one, there is always a prime number p greater than n and less than two times n.
or, properly typeset, without the limitations of DIscourse topic titles:
In 1852, the Russian mathematician Pafnuty Lvovich Chebyshev proved the conjecture for all n>1, in what is now called the Bertrand–Chebyshev theorem. Chebyshev’s proof in terms of binomial coefficients is complex and messy, involving manual inspection to prove the assertion for all n\leq 468, but it gets the job done.
Over the years, further generalisations have been proved including, in 1973 that there is always a prime between 3n and 4n, in 2006 that there is always a prime between 2n and 3n, and in 2011 that as n increases toward infinity, the number of primes between 3n and 4n also tends to infinity.
For what is this useful? Nothing, but it’s cool.