When a writer pictures a certain close-up at a certain moment in the film, he or she may use an insert shot. This describes a shot of some important detail in a scene that must be given the camera's full attention for a moment. Inserts are mainly used in reference to objects, a clock, or actions, putting a key in a car's ignition.
For example: if there's a clock in the room. I, as the writer, might have reason for the audience to get a good glimpse of the clock. I would use an insert shot to suggest the director get a closer shot of the clock at a particular point in the scene.
Note: often; writing important objects in CAPS will convey their importance in the scene and give the director more freedom and a greater feeling of importance. Use inserts only when truly important.