The edit cursor represents the position and size that a new cube will be placed at.
The cursor is a dark yellow blinking cube.
Moving the Cursor With The Mouse
Move the cursor by clicking on the 3D workspace area to position it.
Positioning the Cursor With The Keyboard
Use the left, right, up and down keys to move the cursor around. This works well in conjunction with the Space bar shortcut to add cubes. To move the cursor in Z (front to back), hold down the Ctrl key and either push the up or down key.
(Note that you need to unselect any selected cubes by clicking an empty area of using the Esc key shortcut.)
Resizing the Cursor
Change the snap value on the Edit Menu to re-size the cursor.