This function is used to get the y component of a position "len" pixels from the starting point and in direction "dir". If you imagine a circle around your instance, and then imagine a point anywhere on that circle, to move to that point we need to move the object so many pixels in that direction... so this function (when used with lengthdir_x) gets the position of that point on the circle to be used in code by the instance. See the image below for details:
|len||The length away of the point to return.|
|dir||The direction of the point to return.|
var _xx = x + lengthdir_x(64, image_angle);
var _yy = y + lengthdir_y(64, image_angle);
instance_create_layer(_xx, _yy, "Bullets", obj_bullet);
This will create a bullet instance at (_xx, _yy), which will be 64 pixels from the parent instance in the direction of the image angle.