This quickly changes the date range of the timeline. If you choose a 6 month zoom, it helps you track small changes from month to month. Longer spans like 2 yr and 5 yr are better at showing you trends over time.
The date slider at the bottom of the bottom of the chart lets you set a custom date range until you find the view you want. Simply stretch or compress the shaded area, and click the center area to drag that set range across the timeline.