Let, $\mathbf{A} \in \mathbb{R}^{n \times n}$, $x \in \mathbb{R}^{n}$, and $\mathbf{I}$ be an $n$ by $n$ identity matrix. What does,

$$ \begin{bmatrix}\mathbf{A} \\ \mathbf{I} \end{bmatrix}x,
$$

mean? I see this notation often used in books and no idea what it implies. Thanks!