Come down a long mountain grade and you may notice a hot, acrid smell from the wheels at the bottom. That brake smell is a real warning sign about heat - usually not damage, but a signal to change technique. Here is what it means.
What the smell is
The smell is overheated brake pads - riding the brakes down a long descent builds enormous heat, and the pad material (and any grease) gives off an acrid odor as it cooks; see why brakes smell and how hot brakes get. It is most common when you hold the brakes continuously instead of using engine braking.
Why it is a warning
Hot enough to smell is hot enough to be near brake fade - where overheated pads lose grip or the fluid boils and the pedal goes soft; see signs fluid is boiling. The smell is your cue that the brakes are working too hard and need a different approach or a cool-down.
Use engine braking instead
On a long descent, downshift to a lower gear and let the engine hold the car back, braking firmly and briefly only when needed rather than dragging the pedal continuously - see why brakes overheat towing, which is the same heat problem. This keeps the brakes cool and prevents fade.
What to do if they smell
If the brakes smell, ease off, use engine braking, and if the pedal is fading, pull over safely and let them cool. Usually no damage results from one hot descent, but repeated overheating can glaze pads or warp rotors. For big hills, quality heat-tolerant pads, fresh fluid, and good hardware help.