This is mounted directly under the main mode dial and used to switch from regular single-shot photography to continuous shooting and other modes. In fact, the D7200 has two continuous modes: CL mode runs at a slower speed which you can change in the settings, is useful for longer bursts with slower-moving subjects; CH mode runs at the camera’s maximum speed of 6 frames per second.
Other modes include Q, or ‘Quiet’ mode (it’s not really quiet, but it does delay the mirror return so that you can control the noise), self timer mode and MUp (mirror up) mode. Mirror up mode is used to raise the mirror ahead of the exposure to reduce camera vibration when you take the picture. You might us it for high-precision macro photography, for example.