Properly functioning brakes are critical for safe operation. Always check your brakes are functioning before riding.
Cable actuated brakes will need adjustments as well as periodic pad replacement.
Hydraulic disc brakes are self adjusting as the pads wear, but still require periodic pad replacement.
If the brakes feel spongy and do not apply adequate stopping power, get them adjusted.
If the brakes make a metallic noise when applied, do not ride the bike and be sure to get the brake pads replaced.