According to John Galt, selfishness is both moral and practical. Explain what he means by this?

Ah yes, "Who is John
Galt?"

First of all, you have taken this statement out of
context.  The novel opens with a picture of New York City under a communist regime.  The
city is basically in ruins.  What started as an idealistic, socialistic, egalitarian
society, turned for the worst (as history and literature have so often presented).  John
Galt represents the solution to this major problem - and that is to transform society
back to one of capitalism (and consumerism).  In his idea - nothing should be gained
except that which is earned by a man's own work.

The idea
that "selfishness is both moral and practical" is a hyperbole that responds to realistic
communist societies.  The basic idea behind communism is that
everyone is equal - equal owner, equal sharer, equal partner in everything.  On paper it
sounds great.  In reality, humans are not equal - we just aren't.  Some of us are
smarter than others, some are stronger and work harder, some are more organized and work
more efficiently, etc.  So what happens in communism is everyone gets the same AMOUNT
but does not work the same for it - and as a result - the leadership often rises above,
takes more, works less - and lowers the amount that everyone else gets.  (Remember
Animal Farm?)

Capitalism, on the other
hand, encourages competition.  The idea (and often the reality) is that whoever works
the smartest, or the hardest, or the most efficiently, will in fact get the most out of
their work.  In this way - equality is not measured person-to-person, but
input-to-output, or work-to-benefits.  For how much I work, this is how much I recieve. 
That is where the balance lies.  John Galt's statement then, encourages people to put in
more if they want to take out more.  The selfishness part here is
simply a reminder to be competitive.  But when everyone is competing everyone is
receiving what they deserve - which is the moral and
practical part - according to Galt.