From ROBLOX Wiki

Tool Series: Move on Axis
Use the links below to navigate.
Drag — Move on Axis — Resize — Velocity — Lock — Anchor — Fill with Color — Pick Color

The Axis tool, which is the second button on the toolbar, allows for movement confined to a single axis. This tool lets you move rotated bricks around without losing the brick's angle.

Click on one of the orange arrows to move along that axis.