The Move tool uses SketchUp's sophisticated geometric inference engine to help you place entities in 3D space. The inference decisions, made by the inference engine, are displayed in the drawing area as inference lines and inference points. These lines and points show precise alignment between the move operation and the geometry of your model. Refer to the inference engine topic for additional information.
Locking a move to the current inference direction
Press and hold the Shift key, while the move you are performing is the specific color of an axis, to lock move operation to that axis.
Locking a line to a specific inference direction
Press and hold either the up arrow, left arrow, or right arrow keys, where up arrow equals blue, left arrow equals green, and right arrow equals red, while moving to lock the move to a specific axis.