If you're in something other than ideal light, your camera's built-in metering may screw things up. A cloudy gray can make things gray, metering on a dark spot can overexpose your shot, a patch of stray sunlight can underexpose a shaded photo, things like that. Sometimes you just need to manually push certain shades to white or other shades to black.

That's where exposure compensation comes in. This is the button on your camera that says Ev, and the scale inside the viewfinder that looks something like -2''-1''^''+1''+2. By using exposure compensation, you can automatically make your camera take a shorter or longer exposure or, if using shutter priority, have a wider or narrower f-stop, without having to do the measurement yourself. To make something white, meter on the object you want white and add 1-1.5 stops (2 stops usually blows things out pretty well). Do the reverse for something black (in this case, 2 stops will give you a real black).

In low sunlight, adding 1/3 - 1/2 stop can give you something closer to everyday brightness.