The camera sets the shutter speed and aperture values based on the meter reading. There are various metering modes.
Evaluative metering takes readings from a number of points across the frame. The camera analyses these results to determine any unusual lighting situations. For example, if the light in the centre of the frame is bright, but dark at the edges, the subject might be lit by a spotlight and the camera will adjust the exposure accordingly. Evaluative metering is a good choice for most subjects.
Centre Weighted Average metering also take readings from across the frame, but take more notice of the values from the centre. It can be a good choice for landscape photography.
Spot metering take readings from just a central area (Spot AE Point frame) of the image. This is useful when you want to obtain correct exposure for a specific area of the subject, such as the face in a portrait.