Limits all transformations (move, rotate, scale) to the Z axis.
When you move an object along a plane that is head-on to your view, the object moves along the single available axis shown in the view.
You can also right-click a selected object and choose Transform, which cascades to display the six axis options (X, Y, Z, XY, YZ, ZX). Selecting any one of these is the same as selecting the corresponding toolbar button.