So, the problem is about primitive sets, sets where no element of the set is a multiple of another element. You do have a partially correct intuition here. The canonical example of a primitive set is the set of primes. Buy you can give other examples of primitive sets. For example, you could take the set of primes, remove 2 and 3, and then throw in 4, 6 and 9 into the set. Notice that if I compare this to the set of primes less than 10 which are just 2, 3, 5 and 7, whereas this new set has 4, 5, 6, 7, 9 and so has one additional small element. But the problem in question is one of a series of conjectures which all together say in a certain sense that primitive sets cannot end up being much denser than the set of primes.