Brake pads have a built-in low-pad warning called the wear bar (or wear indicator). It is a simple, clever feature that tells you when the pads are getting thin - no tools needed. Here is how the wear bar works and what it means.
What the wear bar is
The wear bar is a small spring-steel tab attached to the brake pad at a set depth. It is the same thing as the squealer tab. While the pad has plenty of friction material, the tab sits clear of the rotor. As the pad wears down to a set thickness, the tab reaches the rotor and starts to lightly contact it.
How it warns you
When the tab touches the spinning rotor, it makes a distinct high-pitched squeal - often loudest when you are NOT pressing the brake, and quieter when you do. That noise is the pad telling you it is near the end of its life; see brake noise that stops when you brake. It is a purely mechanical, no-electronics warning.
Wear bar vs electronic sensor
The metal wear bar is the mechanical type. Some vehicles instead use an electronic wear sensor that lights a dashboard warning. Both do the same job - alert you to low pads - just by different means. Either way the message is: replace the pads soon.
What to do when you hear it
The wear bar squeal means plan replacement now, before it turns into grinding metal-on-metal - see when to replace pads. Replace pads as an axle set with fresh hardware; the new pads come with new wear bars.