We say that an element belongs to a set if it is part of the set. We use the symbol $\in$.
$$10\in\Z,\;O\in\{T,\Tred{O},K,L\}$$
When an element does not belong to a set we use $\notin$.
$$A\notin\{T,O,K,L\},\;-1\notin\N,\;\pi\notin\Q$$