The camera is evaluating the scene differently. If you're moving the camera then the scene will have a different exposure value. Say you point it out the window and then go to the basement, obviously one is brighter than the other. In manual mode the camera is telling you what results your current settings will get you. EG: If it's at the -2 the photo will be two stops underexposed.
The exposure meter selects an exposure based on the amount of light it "sees".
The combination of ISO, f-stop and shutter speed together define an exposure.
The meter tells you the difference between the two.
When the camera is set to an automatic mode the camera adjusts one or more of the three components of exposure (ISO, f-stop and shutter speed) such that the difference is always zero so the meter does not appear to move around when on auto.
In manual you change the aperture and shutter speed and the camera will hint you as to how exposed the photo will be at the subject you've pointed at.
In Av or Tv you change either aperture or shutter priority (depending what mode) and then you can also change the exposure you want. The camera will then choose a suitable shutter speed/aperture to get the wanted look.
And then in the other modes the camera pretty much does everything for you.