Toe is the angle that the wheels point in relation to the car’s direction of travel. Toe-in is when the front of the wheel is angled slightly inwards; toe-out is when the wheel is angled away from the car.
The front wheels’ toe angles are adjustable; this is essential for setting the tracking so the car drives straight. Rear toe-angle adjustment is only commonplace on multi-link suspension setups and the range of adjustment is less.
Theoretically the ideal toe angle is zero, i.e. with the wheels pointing exactly in the direction of travel. This creates lower rolling resistance, reduces tyre wear and optimises efficiency. Yet most cars have a certain degree of front toe-in: front or rear, it aids straight-line stability.
Toe-out can make a car feel nervous, but this can be used to improve agility; on high performance front-wheel-drive models it also helps reduce understeer.