That symbol means “is an element of”

For example, if I have a set S which is represented by this: {1, 4, 6, 7}, then we can say that 1 ϵ S. This is because 1 is an element present inside the set. In the same vein, we can say that 7 ϵ S.

Now what does the letter R represent? R is used to denote a set of all real numbers. (Real numbers are like 1, 2, 3, 4, 2.75, 3.5, as they can all be expressed as fractions)

So putting these two seemingly abstract symbols together is a fancy way of saying “integer a/b is a real number”.