Reals represents the domain of real numbers, as in x Reals.
x Reals evaluates immediately only if x is a numeric quantity.
Simplify[expr Reals] can be used to try to determine whether an expression corresponds to a real number.
Within Simplify and similar functions, objects that satisfy inequalities are always assumed to be real.
Reals is output in TraditionalForm as .
See The Mathematica Book: Section 1.4.6 and Section 3.3.10.
See also: Element, Simplify, Real, Integers, Complexes, Algebraics, ComplexExpand, PowerExpand.