As everyone else said, it's set notation for all real numbers.

Real numbers include integers, rationals and irrationals such as

, but stop short of including imaginary numbers like

.

Sometimes it will be written

, which means the same thing.

Here are some other ones that you may encounter:

(natural numbers)

(integers)

(rationals)

(complex numbers)

You can also add a superscripted sign to place further restrictions.

e.g.

means positive integers.