what does the leading byte represent for the initial point G? Per SEC 2, the base point $G$ for secp192r1 is as follows:

G = 04 188DA80E B03090F6 7CBF20EB 43A18800 F4FF0AFD 82FF1012
07192B95 FFC8DA78 631011ED 6B24CDD5 73F977A1 1E794811


According to this JavaScript ECDH Key Exchange Demo, when you click the secp192r1 button, the $(x, y)$ coordinates for $G$ are as follows:


x = 602046282375688656758213480587526111916698976636884684818
y = 174050332293622031404857552280219410364023488927386650641


$x$ is equal to what you'd get if you converted 188DA80E B03090F6 7CBF20EB 43A18800 F4FF0AFD 82FF1012 from hex to decimal. Same thing for 07192B95 FFC8DA78 631011ED 6B24CDD5 73F977A1 1E794811.

But what does the 04 at the beginning mean?
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.