I'd always assumed that the existence of the Sith, or at least their being active, brought about an unbalancing of the Force, and by killing the Emperor and returning to the Light side Anakin rebalanced the Force.
So far, so good.
JD said:
Perhaps it doesn't mean balance between Light and Dark, but more of just a peace over the whole thing which is disrupted when people use it for evil. Basically it's more of a philosophical balance, rather than a mathematical balance.
It does mean balance between light and dark. Among the sources which explicitly spell this out are
Cloak of Deception, TCW's
Altar of Mortis, and
The Old Republic: Revan. The problem is when balance between light and dark is replaced by balance between Jedi and Sith. A parity in numbers between Jedi and Sith is not the balance discussed in the films. It is an invalid substitution. The films discussed a balance
of the Force, not a balance in the membership of two organizations. A balance of one type is not automatically the same as a balance of anything else. When you take words out of a phrase and replace them with different words, you tend to change the meaning of the phrase. As depicted in the PT, the restoration of balance to the Force is something that the Jedi hope for as a possible positive outcome in the future. To assume that the Jedi look forward to an outcome of "one Sith for every Jedi" is patently absurd.