How you want to interpret the ending of Electra goes in line with how you choose to interpret the play as a whole. Is it a moral play, or is it a Homeric story of epic heroes doing epic deeds? If you want to go with the latter, it's easy enough to read the Chorus's final ode as a congratulatory "Yes, you did it!" speech. Electra has slain the mighty dragon(s), and now it's time to party like it's 399 B.C.
A great example of a very different interpretation is that of translator and scholar David Raeburn (of Penguin Classics fame). His translation of the final three lines goes a little something like this:
"O seed of Atreus, how much you have suffered!
But now this attack has forced you out
Into freedom. You've come to the ending." (1508-1510)
Raeburn, like many other scholars, interprets Electra as a moral play and, more specifically, as a moral condemnation of Electra and Orestes's action against Clytemnestra. In this translation, the Chorus addresses Electra as "the seed of Atreus," and tells her that she's "come to the ending." The ending of what? Of her mission, sure, but also, quite sadly, of whatever moral scruples she had left.