The (member of) symbol indicates that an expression on the left-hand side of the symbol belongs to or is in the set on the right-hand side of the symbol. Typically the symbol is used in an expression like this:
In plain language, this means that the variable is a member of the set of real numbers denoted by the symbol .
The not in symbol.
The element of symbol describes membership to a set. When reading an equation the symbol can be read as "in" or "belongs to".