A spot meter is a device that measures the reflected light off of an object. It has a very narrow field of acceptance through which it analyzes a specific part of a shot to measure the average reflected light.
By looking through the lens of a spot meter, the DP could measure the light from a specific spot in the scene. Some camera models also have a built in spot meter, so that readings can be directly inputted into the settings.
These readings can help if a DP wanted to employ the zone system to precisely fine tune the exposure of a shot.