What does George Orwells quote in 1984 mean? "those that control the past control the future and those who control the present control the past."

This  answer
is simply pulling apart the quotation.   If you control the present, you are able to present
your version of the past because no one else is in control.  Only you will be able to frame the
past through your lens on history or the past.  If you can control the past, that allows you to
shape the future in your vision framed by what has happened in the past via your version.  For
example, if you see the laws governing pollution control as a good thing, you would present your
view that the laws should continue so that the earth is not poisoned with pesticides or the
chemical runoff of factories.  If you see the laws as a bad thing, your vision of the future
would be that the laws should allow businesses to do as they wish with waste products because
that would be good for their businesses and thus good for the country.   The laws would be
considered government interference and thus up for change.  The future would thus be much freer
for businesses without the regulations controlling what they do.  Now apply the same ideas to
politics itself, and you can see that the quotation applies equally to the business of politics
and the political party or people in control.  They can slant news releases to support their
versions of the future.