Playing / Scrubbing is a part of the Playback settings.
It is important to understand the Live Playback option as it is directly related to what the transport controls affect.
Scrubbing
The above slider allows you "scrub" back and forth through your media.
The part you can move is called the "playhead".
To use this control, click inside the control and, while holding the mouse button down, move the mouse left and right.
If Live Playback is on, and the shot you are editing is live, then the scrubbing will also be shown in your Live broadcast.
Setting the In Point and Out Point
The primary usage of being able to move the Playhead is to set the In and Out Points.
Playing / Transport controls
The buttons shown above allow you to Rewind, Play, Pause, and Fast Forward, respectively.
If Live Playback is on, then pressing the Pause button will pause the media in your broadcast, and so forth.