The wind angle is the direction of the wind in relation to the bullet's flight path.

A wind angle of 0° would be a wind that is blowing in the shooter's face.

A wind angle of 90° would be a wind that is blowing on the shooter's right side.

A wind angle of 180° would be a wind that is blowing at the shooter's back.

A wind angle of 270° would be a wind that is blowing on the shooters left side.

Here is a picture to illustrate this: