When talking about suspension geometry, toe alignment is defined as the difference in length by which the wheels of each axle differ from each other, front to rear, in the straight ahead position. Both front and rear wheels are capable of having toe adjustment. Toe is measured in degrees or fractions of an inch.
Toe alignment is measured from one wheel's center to the other. Toe out or negative toe is when the distance is larger at the front end of the wheels than the back end. Toe in or positive toe is then when the distance is smaller at the front end of the wheels than the back end. Two wheels exactly parallel to each other would have a zero degree toe angle.
Since most BMWs are rear wheel drive, the rear wheels push the front wheels down the road, the front wheels have a toe in alignment. During driving conditions the pushed front wheels will naturally toe out to around zero degrees (parallel). Toe in alignment also increases straight line stability.
Toe setting has one of the biggest impacts on your tire wear, if incorrectly aligned. Too much positive or negative toe will literally scrub your wheels across the pavement in a sideways manner. A toe alignment with a 1/8 of an inch out of spec is the equivalent of scrubbing your tire sideways 14 feet for every mile traveled. Over many miles you can quickly see how this would destroy your tire life.