Trying to understand a simple piece of code: dmd barray

[Dibyendu Majumdar via Digitalmars-d-learn]

Hi,

I am trying to understand 
https://github.com/dlang/dmd/blob/master/src/dmd/backend/barray.d.

Two questions:

1. What does this mean and why is it needed?

line 95: alias array this;

2. The struct has no property called length - but this is 
referenced. Where does this come from?

Thank you

Regards