Exposure refers to the amount of light received by a camera sensor during the capture of a single frame.
If a sensor receives too much light, the resulting image will be over-exposed producing harsh clipping in the bright regions of the image.
If a sensor receives too little light, the resulting image will be under-exposed and much of the detail will be lost in the black areas of the image.
Exposure can be increased in one of two ways: by increasing the aperture size of the lens, or by extending the time the shutter remains open during the exposure of the frame.
Each of these methods has a different effect on the final picture.
Increasing the aperture size will narrow the depth of field of the shot, creating more portions of the image that are out of focus.
Increasing shutter time will increase motion blur, since items in the scene travel further through the frame while the shutter is open.