Russel's paradox notation?

Question

Can someone help me out? I don't really get what "x does not belong to x" means in this:

Answer 1

In this scheme, the elements $X$ of $A$ may themselves be sets. In fact, the set $X$ might contain itself as an element, so that $X \in X$. For example, a set $Y$ satisfying $Y = \{1,2,Y\}$ contains itself.

What we are defining, then, is the set of elements $X$ (of $A$) that do not contain themselves. In particular, if we take $Y$ as defined above, then $Y \notin B$.