Overriding is the ability to control Machines. While an overridden machine cannot be commanded per se, overriding turns a machine from its default state of hostility toward any human to a state of combat friendliness toward the human who overrode it, in that instead of attacking the overrider, it will attack other machines or humans that attack the overrider.
The Nora Brave Aloy gained the ability to override Machines after defeating the Corruptor that attacked Mother's Heart in the aftermath of the Proving Massacre. Upon examining its body, she discovered the component that Corruptors used to slave the CPUs of other machines. She attached the component to her spear and began to use it to override Machines. At first, she was only able to override the weakest machines: Watchers, Striders, Broadheads and Chargers. However, after exploring and downloading override codes from successive Cauldrons into the component, she gained the ability to override successively stronger and more dangerous Machines, culminating in the three strongest and most dangerous machines: Thunderjaws, Stormbirds and Rockbreakers.
The wandering researcher Sylens learned about the override ability by watching Aloy through her focus. Presumably, he was able to advance the skill to actually being able to command Machines, as he was able to bring three overridden Ravagers with him to aid in his rescue of Aloy from the Eclipse leader Helis after he captured her.
The ability cannot be used on an attacking Machine, nor can it be used on Corrupted Machines. In order to be successfully used, the Machine must be either unaware of the overrider's presence or physically restrained, e.g. by ropecaster lines. Furthermore, damaging the overridden Machine above a certain threshold will revert it to its default hostile state.
During the main mission The Mountain that Fell, Aloy will discover the master override in GAIA Prime, which is essential in defeating HADES as it can purge the Extinction Protocol. The Master Override is then fitted near the blade of Sylens' Lance.