The LifeLine is the graph at the top of the Timeline page and shows the entire life of the current search. The relative height of its bars (Y-axis) correspond to the relative number of events that occurred in that period of time (X-axis).
The number of events for the life of the current search can be seen in the upper-right:
In the highlighted area of the image below, you'll see the LifeLine has a gray bar running across it. By default this bar will cover the most-recent 10,000 events.
This gray bar is the TimeSlice Selector and can be adjusted to narrow down the in-scope events that are displayed in the TimeSlice below it.
To adjust the TimeSlice Selector, mouse over the sides of the box until you see the pointer change to double arrows. Click and drag to adjust focus/zoom your view.
The example below shows what this could look like: