(Geometry) Proof type questions 

Can someone please explain to me the given question and proof? otherwise I might just have to end up dropping my maths course because unfortunately I'm not understanding anything from my teacher. I'm not really 'seeing' the concept, if that makes sense. 
Regards, 

The thing you might be tripped up on is the 180. The 180 comes from the 'line' DE, which is a straight angle, you know, a 180. And since 180 is a constant, subtracting either $\angle CBD$ or $\angle ABD$ you'll end up with the same number.
