Shot reverse shot is a filming technique used when two or more people are speaking to each other in person. The technique is explained below:
A camera is placed behind the first actor who is often barely in the shot and focuses on the second actor in front of him. That actor will then say his lines one after another, making sure to leave a few seconds gap after each one.
The camera is then moved so it is behind second actor and focuses on the first (please note the following of the 180° rule). That actor then says his lines, also leaving a few seconds gap after each one.
The two septate shots are then edited to make one single tape of the whole conversation playing out in order and thus creating an illusion of verisimilitude.
This filming technique requires only one camera, making the scene a lot cheaper to shoot. It also allows you to shoot half of the scene even if only one actor is in on that day. Simply get an actor who looks like the absent one from behind and no one is the wiser.
However, the actor being filmed may have to say his lines without the other actor joining in, meaning the first actor has to be be aware of how his colleague will act his lines in order for the scene to look and sound convincing. Another problem is that the scenery around them must be continuous. Let's say the two actors are talking next to a road; a car is seen driving in their direction, the second actor is then filmed and so the car must be seen driving past him. For this reason directors often have to request roads to be closed on the day of filming which can be expensive. If there was a car involved in the scene it would have to be carefully and quite painstakingly timed, especially if the car was to be in more than two shots.
Nevertheless it is a technique that is very common in the film industry. Below is a clip from BBC's The Trip which often uses this technique.