Select the item you want to move by clicking it, then click the four outward-pointing arrows on the editor toolbar. This will put your object into move mode (by default you will be in move mode, so you may not see a change).
You will see the red, green and blue move widget at the center of your selected object. If you don’t see any widgets, click and drag the background to rotate the camera until you can see it.
When you hover over one of the colored arrows it will turn yellow. This shows you the direction you will be moving the object. Click and drag to move the object back and forth in the chosen direction.
You will see there are also three colored flat squares at the center of the widget. These allow you to move the object in two directions at once as if it was stuck to a flat surface (this is what the square represents).
Practice using this advanced mode by clicking and dragging the red square when you have an object flat on a marker. You will notice you can move the object across the marker, but it won’t move up or down off the surface, no matter where you move the camera.