This is just a little set notation, ∈ means 'is an element of' the set after the symbol. So for example, ∀ n ∈ N n^2 + n + 41 means 'for every n in the set of real numbers, n^2+n+41'. Reading the linked story, that's a proposition for the definition of prime numbers setting up for a proof. You usually learn set notation, proofs, induction, relations, and most of the stuff before probability I scanned in the Table of Contents there in a discrete math course. Many also cover probability, but ymmv.