In _get_reward() function,what's the mean of setting the reward about plane_heading and plane_heading2?

[8acemak1rkey]

if self.plane_heading>180:
deger_1 = (self.plane_heading-360)
else:
deger_1 = self.plane_heading

    if self.plane_heading_2>180:
        deger_2 = (self.plane_heading_2-360)
    else:
        deger_2 = self.plane_heading_2
    self.reward -= abs(deger_1-deger_2)/90

I don't know what's the meaning of this reward setting.
I don't know what's the meaning of this code.
What do these equations mean？
Thank you very much for your reply！