Rule of Thirds

The rule of thirds applies to the composition of a shot. The frame is split into 9 equal sections.

Like this -

This acts as a reference point which then acts as a guide for framing the shot. 

Points of interest should occur 1/3 or 2/3 of the way up or across the frame, not directly in the centre. This is because the eye does not automatically go to the centre of the screen, so if something is in the centre it makes it look strange to the audience.

Here's an example-

This is taken from The Vow (2012) starring Rachel McAdams (above) and Channing Tatum. In this shot we can see how the rule of thirds is used as she is not directly in the centre, making the shot feel more natural.