This symbol could indicate a serious Brake problem or a minor one, and anywhere in between. However, the light may also simply mean your emergency or hand brake is engaged. So if you see this light come on in your car, first make sure your emergency brake or hand brake is not engaged. If it is, simply disengage it, but if it's not, bring your vehicle to a stop as soon as you can. To continue driving with defective brakes es extremely dangerous, for obvious reasons.
There are multiple parts of a the braking system that could fail, some of which, such as brake pads, need to be regularly changed. Failure to do so can lead to very expensive repairs. A properly trained technician will be able to let you know exactly what part of your braking system is failing and make the necessary repair.