A brake pedal that pulses or throbs as you slow to a stop - most noticeable at low speed - is a common complaint. It points to the rotors and how evenly the pads grip them. Here is what causes low-speed pedal pulsation.
Rotor thickness variation
The usual cause is disc thickness variation (DTV) - the rotor is slightly thicker in some spots than others, so the pads grip harder and softer as it spins, pulsing the pedal; see what is brake judder and pedal pulsation causes. At low speed each revolution is felt distinctly as a throb.
Runout from a dirty hub
Excess runout - the rotor not spinning perfectly flat, often from rust or debris trapped at the hub - also causes a pulse and can grow into thickness variation; see rotor runout. A rotor installed on a dirty hub or with unevenly torqued lugs pulses even when new; see new rotors vibration.
Low speed vs highway
The same rotor problem shows differently by speed: a distinct throb you can count at low speed, and a higher-frequency shudder at highway speed; see brake shudder at 60 mph. Both trace to the same rotor thickness or runout issue.
The fix
Mild deposit-based pulsation can sometimes be cleaned up with a hard re-bedding, but established DTV means resurfacing or replacing the rotors - see resurface vs replace. Clean the hub, torque the lugs properly, fit rotors as a pair with quality pads and fresh hardware, and bed them in.