what does the leading byte represent for the initial point G?
But what does the 04 at the beginning mean?


It means 'this elliptic curve point is specified in uncompressed format', that is, the $x$ and $y$ coordinates are given explicitly.

The other alternatives are :


02, which means 'this is a compressed point, where we give the $x$ coordinate explicitly; of the two possible $y$ coordinates that are compatible with that $x$ coordinate, select the one with the 0 lsbit.
03, which is the same, except you select the $y$ coordinate with a 1 lsbit.


The compressed formats are about half as long (saving space), but requires more computation (a modular square root) to use if you perform an operation that requires the $y$ coordinate.