What does ℜ symbol mean? I came across the symbol $\mathfrak{R}$ in the Kalman introduction: Kalman Filter Intro

$$
x\in \mathfrak{R}^m 
$$
It was used in this context:

$$
    x_{k} = Ax_{k–1} + Bu_{k – 1} + w_{k – 1}
$$
What does $\mathfrak{R}$ stand for?
It means $x$ is an $m$-vector of real values. $\mathfrak{R}$ itself is the set of real numbers.