You can use the scrollbars (located on the right and bottom of the timeline), use the mouse wheel anywhere over the timeline to scroll horizontally, or hold down the Shift key while using the mousewheel to scroll the timeline layers vertically.
Zooming directly affects your editing accuracy and your ability to move quickly along the timeline:
When zooming in, video clips display more thumbnails and audio clips display increasingly detailed waveforms, allowing you to do precise edits.
Zooming out allows you to scroll faster along the timeline. Zoom can thus be used in conjunction with scrolling to move quickly between distant points of your project's chronology.
You can zoom in and out by:
Holding the Ctrl key while using the mousewheel over the timeline
Using the View ▸ Zoom In and View ▸ Zoom Out menu items
Dragging the zoom slider on the left of the ruler
Using keyboard shortcuts such as Ctrl+- to zoom out or Ctrl+= (Ctrl++ on the numeric keypad) to zoom in
To get a better overview of your timeline, you may use the View ▸ Best Fit menu item or click the Zoom button on the left of the zoom slider.
When using the mouse wheel to zoom in and out, remember that the zoom is not relative to the position of your mouse, only to the position of the playhead on the timeline.
Using your mouse's scroll wheel, you can also warp instantly from any zoom level to another in a single click. To do that, click with your mousewheel anywhere on the zoom slider.
You can change the position of the playhead by clicking anywhere on the ruler or on the timeline.
When clicking on the timeline:
If a clip is under the mouse cursor, it will replace the current clip selection (see the Selecting multiple items section for more detailed instructions).
If there is no clip under the cursor, all selected clips (if any) will be deselected.
When clicking on the ruler, only the playhead's position changes; clip selections are not affected.
Scrubbing is a way of moving the playhead that allows you to:
Search for a specific position in time by approximation/trial and error.
Review repeatedly the visual motion in a scene by moving back and forth in time.
To scrub, click on the ruler and move the mouse with the button still held down.
You can also scrub by dragging the black vertical line that stems from the playhead and spans across the timeline.