What is the final interpretation of the word “equal?”

All animals are equal, but some are more equal than
others.  This becomes the last commandment and it is an interesting one.  The concept of
"equality" is something that Orwell plays with quite a bit in his work.  It ends up
being a condition that the reader is left to assess and debate in terms of how it can be
used to both inspire people to revolutionary causes, but also be used by those in the
position of power to actually advocate that which is its
antithesis.

The fundamental call to "equality" would be
seen in the Animalism commandments that are adopted after the revolution.  The events
leading to the revolution represented a fundamental disconnect between animals and
humans, where an "unequal" relationship of power existed.  Animals were secondary to
humans and this hierarchy served as an abusive condition in their lives.  To this end,
once the revolution was waged and Jones was sent packing, the animals come to accept the
seventh commandment of animalism:

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7. All animals are
equal.

It was in this that
the initial condition of "equality" was
introduced.

However, as the novel advances and the
predicament of the animals becomes more complex, there arises a need of leadership. 
Since the pigs were the first to read and the first to fully grasp the message of Old
Major, after all they did sit in the front and heard every word he spoke, the pigs were
seen as the leaders of the farm.  With leadership, a fundamental hierarchy was
developed.  As Napoleon raised the pups to become his secret army and form of
enforcement, the dogs were seen as second in command, the brawn to the pigs' brain. 
Over time, the pig population increased as well as the dog population, as it became good
to be either one.  For the other animals, life did not change.  Every time it was
insinuated that there was an imbalance of power, Squealer would spin how everyone works
for everyone's benefit or Napoleon would simply quash it through force.  In the end,
everyone believed in the theoretical principle of equality.  With this as a smokescreen,
the pigs and dogs were able to enjoy privileged status and be able to "live it up" while
ascribing to equality as a pretext.

It is for this reason
that the addition Squealer maks to the last commandment becomes so
telling:

All
animals are equal, but some animals are more equal than
others.

It is a great play on
words and theory, a stroke of genius that brings out how those in the position of power
see themselves as fundamentally different than those who are on the outside of power. 
This becomes a warning about the nature of equality, in that one must strive to fight
for it to ensure that it is not one that is abused to actually perpetuate a system that
is unequal, a realm where "some are more equal than others."