The set of real numbers symbol is the Latin capital letter “R” presented with a double struck typeface. The symbol is typically used to express that a variable is a member of the set of real numbers:
$x∈R$
In plain language, the exppresion above means that the variable $x$ is a member of the set of real numbers.
Symbol  Format  Data 

ℝ  Unicode 

$R$  TeX 

SVG 