Brake pads wear out during use. All disc brakes always have an inner and an outer brake pad. The inner one (towards the wheel) is always fixed, while the outer one can move back and forth. After a certain time, the inner lining must therefore be readjusted and slightly adjusted towards the brake disc in order to ensure an even and safe braking process again. To do this, readjust the large adjusting screw on the inside of the brake calliper accordingly.
It is also possible that the brake pads have reached their run-off / wear limit and need to be replaced with new ones.
On vehicles with drum brakes, you can readjust them using the Bowden cable and the locking screw.