Put simply, wheel Alignment, or tracking as it is sometimes known, adjusts the direction in which your vehicle's wheels are pointing.
Over time, a combination of driving forces and the general state of our roads, will result in the wheels becoming misaligned. If left unchecked, the car's tyres will suffer significantly higher wear rates. In addition to this, the car can pull to one side, something nobody wants during an emergency stop.
Generally speaking, if the wheels are pointing away from each other, it is known as 'Toe Out'. Towards each other, is rather unsurprisingly known as 'Toe In'.
Depending on the make and model of your car, there could be several other adjustments to be made during wheel alignment.