Trail of a bike determines the steering wheel of a high degree.
Because of this position of the fork goes each wheel act as a caster: it follows the direction of the “upper construction.”
The drive force always pushes a vehicle straight ahead, in which this position ensures that also the front wheel remains in the straight ahead position.
On bikes often notice this effect because the steering wheel remains straight when it is released.
A larger “trail” gives a better directional stability, but makes it difficult to curve.
In various cycling is another “trail”, depending on the use.
Many fast straight runs: greater trail, many bends (town): minor trail.