Equation of a Plane

In 3-dimensional space with $x$ , $y$ -, and $z$ -axes that intersect at right angles, the equation

$ax+by+cz=d$

where at least one of $a$ , $b$ , and $c$ are non-zero,   describes a plane .

Examples:

$z=3$ describes a horizontal plane $3$ units above the $xy$ plane.

$x+y=1$ describes a plane parallel to the $z$ -axis, which intersects the $x$ - and $y$ -axes at $\left(0,1,0\right)$ and $\left(1,0,0\right)$ respectively.