In a drum brake system, the brake drum is the part the friction acts on - the drum-brake equivalent of a rotor. It spins with the wheel and provides the surface the shoes push against. Here is what a brake drum is.
What the drum does
A brake drum is a round metal housing bolted to the wheel hub that rotates with the wheel. The brake shoes sit inside it, and when they push outward against the drum's inner wall, the friction slows the wheel - see how drum brakes work. The drum is to a shoe what a rotor is to a pad.
Drum vs rotor
A drum is a closed cylinder the shoes press outward against from the inside; a rotor is an open disc the pads squeeze from both sides - see drum vs disc. Drums hold heat more and can fade sooner under hard use.
How drums wear
Over time the inner friction surface of the drum wears larger in diameter and can develop grooves or go out-of-round, causing a pulsation much like a warped rotor. Drums have a maximum diameter stamped on them, similar to a rotor's minimum thickness; see thickness limits.
When to replace
Replace a drum that is worn past its max diameter, deeply grooved, cracked, or out-of-round - resurfacing is possible if it stays in spec; see resurface vs replace. Do drums as an axle pair. Keep front discs fresh with quality pads and hardware.