In The Miracle Worker, what does Annie mean by "words, why, you can see five thousand years back in a light of words"?

I can see your conundrum if you take these words out of
context.  It's best to look at the context if you would like to find out the full
meaning.  First, this is spoken by Annie at the moment when she expects the Kellers to
return to retrieve Helen from the little cottage where Annie has been able to teach
Helen alone for a couple of weeks.  Annie is upset that they are demanding Helen's
return before Annie has succeeded in imparting the precious gift of words and language. 
In her frustration Annie exclaims:

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I wanted to teach you--oh, everything the earth
is full of, Helen, everything on it that's ours for a wink and it's gone, and what we
are on it, the--light we bring to it and leave behind in--words, why, you can see five
thousand years back in a light of words, everything we feel, think, know--and share, in
words, so not a soul is in darkness, or done with, even in the grave.  And I know, I
know, one word and I can--put the world in your hand--and whatever
it is to me, I won't take
less!

In full from this
paragraph, it becomes clear that Annie wants to open up the world for Helen, ... not
just the gift of language but fleeting moments in time and nature, history, feelings,
... in order to lift the darkness in Helen's life.

More
specifically, in regards to the partial sentence you mention, Annie is speaking about
history.  How (oh how!) would Helen EVER know anything about history without
understanding words?  And yet the use of language would allow her to "see five thousand
years back" by the simple use of words!  And oh how the stubbornness of Annie is evident
as she wants it to be Annie and Annie alone to teach it to Helen! 
Such frustration is evident here!