definition
The deliberate slowing down of the speed of the film so that everything is seen more slowly than in real life.
director's intention
Slow motion is used to emphasize the detail and to allow the audience time to think about the emotion of the characters.
impact on audience
The audience observing a slow motion scene are drawn to the detail and the use of this technique means it makes important scenes more obvious. By slowing down the action, the audience has time to appreciate the detail and it creates more time for the viewer to consider the emotion or the message from that particular scene.
examples
In this Snowpiercer example, an extremely violent scene is slowed down so the audience can absorb the extreme violence of the battle and notice who is fighting against who. The slow motion gives the director time to include some close ups of actor's faces and thereby conveys emotion as well as detail.