{X∈Z x=2k+ 1, k∈Z}, you can understand it this way. First of all, X∈Z means belonging to an integer set, then in the integer set, x must satisfy x=2k+ 1, and then k belongs to the z integer set. You can try. If k=2 and x=5, this is an odd number. All odd numbers x can be written as 2k+ 1, provided that k is an integer. So | actually means something like "and"