I've something to add here. If you fluid is low.. DO NOT TOP IT OFF! You should never have to top off brake fluid unless you've bled some out, or there's a leak somewhere.
You can top it off, AFTER you check ALL your brake pads, as well as your lines and calipers for leaks. The brake systems is supposed to be completely sealed. If there's a leak, fix it. If the fluid is down and there are no leaks, then your pads are likely very worn, or someone bled the brakes and didn't re-fill properly.
As pads wear, the pistons extend and leave space in the calipers. The fluid from the reservoir fills the calipers. As the calipers fill, the reservoir (obviously) drains, setting off the "brake" light. This is by design.