How do I know what is wrong with my brakes?
How thick are the pads? Are these rim brakes or disk brakes? For rim brakes, the block (rubber) will get worn away over time so you should adjust the cable occasionally, but well before the block is so thin that the shoe (the metal backing) starts to scrape on the rim.

For disk brakes it's essentially the same but the pad is thinner and made of harder material. The manufacturers usually give a number how thin the pad can become, something like 0.7 mm.