Answer
In three-space, $y=3$ represents the plane parallel to the $xz$-plane that passes through the point $(0,3,0)$
In three-space, $z=5$ represents the plane parallel to the $xy$-plane that passes through the point $(0,0,5)$
The pair of equations $y=3, z=5$ represents the line created by the intersection of the planes $y=3$ and $z=5$.
Work Step by Step
As mentioned in the book, in three-space, in general, if $k$ is a constant, $y = k$ is a plane parallel to the $xz$-plane and $z=k$ is a plane parallel to the $xy$-plane, so we arrive at the conclusion that $y=3$ is a plane parallel to the $xz$-plane and that $z=5$ is a plane parallel to the $xy$-plane.
However, to fully describe the planes, it is not sufficient to say that they are parallel to some other plane, because there are an infinite amount of planes parellel to some other plane. To resolve this issue, we can indicate a point belonging to the planes, since there is only one plane parallel to another that passes through a certain point.
In three-space, the plane $y=3$ contains all points that are of the form $(x,3,z), x,z\in \mathbb{R}$ so, in particular, it contains the point $(0,3,0)$. The plane $z=5$ contains all points that are of the form $(x,y,5), x,y\in \mathbb{R}$ so, in particular, it contains the point $(0,0,5)$.
We can then describe the planes as shown in the answer.
The representation of the pair of equations $y=3, z=5$ can be thought of as the intersection of the planes that make the pair of equations, that is to say, the intersection between the planes $y=3$ and $z=5$. That intersection creates a $\textbf{line}$ that contains all the points of the form $(x,3,5), x\in \mathbb{R}$.
