Explain staging

[Stephen Montgomery-Smith]

I have not been following discussions recently.  There is this new thing
called staging.  I don't seem able to find the original emails where
this was introduced.

Is there an email or a website which explains in some detail how staging
works?  It is badly messing with a port I am developing.  I can fix it
by setting NO_STAGE=yes, but I would like to get a better idea of why it
is failing.  The failure is during the build process.

Sorry for not following closer.