Manual is one of three common User Control Shooting Modes. In Manual Mode the user has total control over the exposure variables of ISO, Shutter Speed, and Aperture. 
When photographers begin to master exposure variables, Manual Mode is no longer an intimidating camera feature, but a desired one.
In Manual Mode the user sets the desired Shutter Speed, and Aperture value, as well as selecting the desired ISO. This is most often accomplished with different buttons, or dials on the camera, so refer to your user guide on how to adjust both Shutter Speed, and Aperture at the same time.
Shooting in Manual Mode has many applications; most often when the photographer needs more control over the exposure. Some examples include low-light settings such as gymnasiums, or rapidly changing lighting scenes such as walks through nature (in and out of sunlight).
CONSIDERATIONS
- REFER TO YOUR MANUAL: Find out how to make Shutter Speed and Aperture adjustments when in Manual Mode.
- PUSH LIGHT LIMITS: Manual Mode allows users to force the camera to reach the extremes of light gathering for an exposure in low-light situations.
- METERING, BUT NO LIGHT METERING: In Manual Mode light metering is still active in the sense that the meter takes readings and displays the Exposure Value on the scale in your viewfinder. But the camera does not use this value to create the exposure since the user is setting all the variables. This does help photographers, however, to see if their settings will result in an image that is too dark, or light.
- ZOOM LENSES: remember that if you have a zoom lens that changes aperture with the zoom extremes, you may need to adjust the shutter speed when zooming in or out to have a well-balanced exposure.