You may want to move a note a little to the right or left for aesthetic reasons, or to make more room for an accidental, or to make two notes a little farther apart to keep lyrics from running together. The Spreader tool will add to the space between two notes or other symbols; the Shrinker tool will take away space. Place the tool just before the note you want to move and click the mouse. Each click moves the note one pixel, or you can just hold the mouse down. As a shortcut, pressing command-";" will change the spreader tool to the shrinker, and vice-versa.
(If justification is turned on the effect of these tools may not be obvious, since each line of music will be constantly readjusted to come out even at the right side of the page. It's best to have justification turned off in the Score Menu whenever you're editing music, and turn it on only when you need to print).
Remember that you can also change the note spacing of the entire score with the "Set note spacing" command in Score Options, and you can compress or expand the whole contents of a bar by dragging the barline left or right.