How does Jared Diamond define east-west axis and north-south axis in his book Guns, Germs, and Steel?

Whether you agree with Diamond or not, his theory is
interesting in providing a likely explanation of the development of our species.  It is
what we now call 'big history' (the big picture), rather than traditional history which
concentrates on documented sources.

As other's have pointed
out, Diamond theorizes that human civilization is strongly a product of geography.  In
this way, Diamond's ideas can be said to be deterministic.  For
example, topography and climate will guide (or separate) human groups, either aiding or
limiting the spread of ideas and developments.

The argument
goes that Eurasia with its open plains and east-west orientation permitted the spread of
people's and agriculture.  By contrast, the other continents with their north-south
orientation meant that topographical barriers (Rockies, Andes) and climatic bands
(Amazon Rainforest, Sahara) limited the spread and diffusion of migration and therefore
ideas (and of course microbes!).

I hope that helps enough
for you to understand Diamond's thesis a little better.