That light has a dual purpose. It is connected to the parking brake, and it's tied into the low fluid sensor in the brake fluid reseviour.
When your fluid is low, it sloshes around during braking and accelerating and trips the light.
Don't just add fluid though. Fluid doesn't evaporate, so if it dropped down it's either due to a leak or your caliper pistons are extended out far due to worn pads.
Ideally, with fresh pads and the fluid filled to the MAX fill line, the light should start to come down when the pads are nearly worn down. Doesn't always work like that, but that's why they do it like that