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".