'In/Out' points can be set on the timeline allowing you the ability to view part of the timeline only.

Only one set of points can be set at a time. The timeline outside the points will be greyed out.

If the points are set and Looping is on then the media will only loop between the points.
The points are set from the keyboard and can be set on the fly.

I = In point

O = Out point

Apple / Ctrl T = Toggle points on/off

C = Clear points