Ackermann is the steering geometry that makes an RC car's inside front wheel turn at a sharper angle than the outside wheel, so toe changes as the wheels turn. It's adjusted at the steering rack.
You change Ackermann by moving the ball stud to a different hole on the steering rack or the steering arm, or by adding and removing spacers under the rack ball studs — both angle the steering links relative to the chassis. More Ackermann means more toe-out as you turn, which gives a smoother steering feel and more off-power steering. Less Ackermann means more on-power steering and a twitchier, harder-to-drive car. It’s a fine-tuning change, not a first move: sort camber, toe and ride height first. Full-size cars carry Ackermann geometry too, but there it’s designed in, while on an RC car it’s a hole choice you make with a ball-stud wrench.
Ackermann is what makes steering feel progressive instead of snappy, so getting it wrong leaves you fighting a car that's either lazy on entry or too sharp to drive for a whole main.