The directional arrows on your tyre indicate the correct mounting direction, when there is one. This is important when the tread is designed for a particular feature, such as traction or speed. Some tyres will have no directional arrows, indicating that the tread is not directional and you can mount the tyre either way. Other tyres will have arrows pointing both ways accompanied by a word such as GRIP or SPEED, meaning that depending on which way you mount your tyre you will achieve one characteristic or the other.