A two-shot is any shot with two people in it. It's a very simple idea, but it is very essential to filmmaking as it can easily characterize two characters and their relationship.
In this shot from Sidney Lumet's Dog Day Afternoon, Sgt. Eugene Moretti looks at the invasive Agent Sheldon with scorn for his interference. Even without knowing the plot of the film or the context of the shot, the audience knows that the relationship between the two is strained by the body language of the actors and and the mis en scene of the shot.