If brakes make a consistent high-pitched squeak, even with gentle braking, the brakes are likely completely worn out. Some noise from the brakes during operation is considered normal, as various driving conditions cause different noises.
When a car is in operation and the brakes are applied, the brake pads and rotors press against one another with significant force, causing them to wear down. When they deteriorate past a certain point, they squeal due to a mechanism built into most current brake pads for consumer safety.
There are other reasons brakes may squeak, but none of them are cause for concern. When driving in an area that is very dusty or sandy, the brakes often squeak. Fine dust or sand particles often gets between the brake pad and rotor when the brakes are applied, and these particles cause a squealing sound when pressed against the metal between the brake rotor and the break pad.
Another common reason brake pads squeak is due to moisture. When it rains, or is especially humid, brake rotors may build up small amounts of rust. The rust and small amounts of moisture on the brake pads and rotors cause a squealing sound. Additionally, some newer cars use semi-metallic brake pads that are harder than traditional brake pads and can vibrate against the rotors, creating a squeal.