Answer
In 3D $x^2+y^2=4$ is a $\bf cylinder$ of radius $2$ centered on the $\bf z-axis$ extending indefinitely in the $z$ direction.
Work Step by Step
$x^2+y^2=4$ reminds us of the formula for a circle in 2D:
$(x-h)^2+(y-k)^2=r^2$
where $(h,k)$ is the center and the radius is $r$
Since $h=k=0$ the center is $(0,0)$ and the radius is $r=\sqrt4 = 2$
So in 3D $x^2+y^2=4$ is a $\bf cylinder$ of radius $2$ centered on the $\bf z-axis$ extending indefinitely in the $z$ direction.
