If your brakes have suddenly started smoking then it's likely your brake calipers are sticking and causing your brake pads to remain in contact with the brake rotors.
If the brake pads do not fully release it can cause your brake pads to wear against your brake rotors which then can cause your brakes to smoke.
That friction creates heat and is what stops your vehicle when you press your brake pedal down.
It's normal for brake rotors and brake drums to get somewhat hot because of the friction used to stop the vehicle.
However if your brake drums or brake rotors are getting really hot then it's an indicator of a problem.
It's unlikely that your brakes would catch on fire but if your brakes are smoking you should stop the vehicle and get it checked.
If you have to you should get the vehicle towed home or to a shop and get the brakes checked.
There's also another possibility of brakes smoking which has happened to me before.
I had set the parking brake when parked on a hill at a store and forgot to release the parking brake and then I smelt something burning and my brakes were smoking.
I then realized I had forgot to release the parking brake so check that to be sure you didn't leave the parking brake on.
Brake calipers can stick or your brake rotors may be warped causing your brake pads to rub against the rotors and causes them to smoke.
Or if you ride your brakes too much going down a hill then you may notice smoking brakes.