In order to understand the function of brake shoes, you first need to know the basics of drum brakes. Before disc brakes became popular in the 60s and 70s, drum brakes were used for all four wheels of a car. On most newer vehicles, they are installed on the rear wheels only and combined with front-end disc brakes.
Drum brakes involve using friction to slow down the drum, which is attached to the wheel. This is not unlike the way that brake pads squeeze rotors in a disc brake setup. The brake shoes are essentially serving the same purpose as brake pads, but the arrangement is slightly different. Rather than squeezing the flat side of the wheel, the brake shoes are located inside the brake drum and push outward to bring the wheels to a stop.
There are two brake shoes in each drum that sit on opposing sides. When you press the brake pedal, it triggers hydraulic pressure to force the brake shoes outward, against the interior wall of the drum. The friction that stops the car can also wear down brake shoes over time, and they will eventually need to be replaced. There is a friction material that is bonded to brake shoes, which will allow shoes to last longer than brake pads, but the length of time before they need to be replaced can depend on how often and hard you drive your vehicle, road conditions and your climate. The owner's manual may also indicate recommended intervals to inspect or replace the parts.
As brake shoes begin to show signs of wear, you may notice that the brake pedal travels farther before you feel its effects. It can also feel "soft" when you step on it if the brake shoes are nearing their end. However, this may also indicate low brake fluid, a leak in the system or other issues.
If the brake shoes are the root of the problem, it may be that they need to be adjusted. Drum brakes typically include automatic adjusting mechanisms, but if this fails, the brake assembly may need to be taken apart to correct the issue. Some vehicles are built with a small access window so this can be done without removing the wheel or drum.