In the field of set theory, ∈ (written ∈ in HTML) is shorthand for "element of". It just means that the thing to the left is a element of the set on the right. This should not be confused with lowercase epsilon (ε), which is short for empty string. It is not uncommon to see ε ∈ A, where A is a set.

Just in case your browser can't render the element of symbol or epsilon, here's a little ACSII art for you.

 element of      epsilon
   ______         ______
  /              /
 |_______        \____
 |               /
  \______        \______