Colorimeters (AKA color readers or color matchers) are devices that measure the objective value of a color, facilitating paint matching for touch-ups or expansions. The human eye can be fooled by any number of contextual factors into misreading a color. What kind of light is in the room, how dilated your pupils are, what colors are nearby, and even what kind of mood you're in might influence how you see a certain hue at a given moment.
Since there are thousands or even millions of colors used in commercial products like paints and plastics, no human can reliably identify the exact brand of every shade. Color readers take the guesswork out of this process by measuring with mechanical precision the absorption of various wavelengths of light created by the color in question.
Most colorimeters are dedicated devices with their own light source. To get a reading, apply a reader directly to a painted surface, or to whatever area for which you want to identify the color. Some of these gadgets communicate with an app on your phone, and a few even recommend the most popular commercial paint brands that are near or perfect matches.
If you want to add a coat of paint to a surface, or paint a nearby area, but you don't know what kind of paint was used for the first round, use a colorimeter to get the maximum possible precision for your project.