There are a few variations on this one, but the basic idea is to have a comfortable shot of two people. Often used in interviews, or when two presenters are hosting a show.
A "One-Shot" could be a mid-shot of either of these subjects. A "Three-Shot", unsurprisingly, contains three people.
Two-shots are good for establishing a relationship between subjects. If you see two sports presenters standing side by side facing the camera, you get the idea that these people are going to be the show's co-hosts. As they have equal prominence in the frame, the implication is that they will provide equal input. Of course this doesn't always apply, for example, there are many instances in which it's obvious one of the people is a presenter and the other is a guest. In any case, the two-shot is a natural way to introduce two people.
A two-shot could also involve movement or action. It is a good way to follow the interaction between two people without getting distracted by their surroundings.