Lets say you have some background video and one item over top of it. You can click on it in the timeline and then move it around in the Canvas with the mouse (you'll need to select the top left pull-down menu in the Canvas and select Image + Wireframe first) but there's a easy way to move it around (nudge) by very small amounts to get it in just the right position.
With the item highlighted in the Timeline and the Canvas the frontmost active window hold down the Option key and tap any Arrow Key. You'll see your element moving in small amounts for precision placement.
Hold down Command + Option and then tap an Arrow Key for really small movements. So small in fact that you may not be able to detect the movement unless you zoom in on the Canvas (Command + + (plus) or - (minus) with it the active frontmost window) a bit.