Yoda is referring to the ultimate victory over the Sith.
I refer to the same episode, but through multiple links to obtain a clear reading.
For uncropped, somewhat clearer video feed (correct episode, but incorrect episode number in the title):
For tweaked but comprehensible audio feed:
This scene, which happened shortly before the one you quoted, contains the answer. After successfully surviving Darth Sidious' attempt to break him, Yoda is brought back to consciousness by a Force Priestess, who then told him that he will receive training to retain consciousness after death from Qui-Gon Jinn.
I can't quite hear what the Force Priestess next said (I blame the Force afterlife for giving everyone Dolby Surround Sound), but I found out from another answer on this website. Quoting:
Like us, you shall learn to maintain your consciousness after death.
Enlightenment, sprit, balance...
There is another Skywalker...
Judging by the whitening out of the scene, the background sound effects, the widening of Yoda's eyes, and the above lines, I believe the Force Priestess revealed to Yoda a vision answering exactly why he is granted this gift:
- To serve the role he will play in ensuring the fulfilment of the prophecy for Anakin Skywalker, the Chosen One, to defeat the Sith once and for all, bringing the Force back into balance.
The vision showed Yoda a grim fate for the Chosen One, yet also showed hope beyond hope. Open to him (and Obi-Wan), this path revealed that unknown to the Sith is. Through this path, victory they may yet find, yet it may also not, for the future is ever shifting. Victory over the Clone Wars is no longer important. What Yoda seeks now is not victory over the Separatists, but victory over the Sith and liberation of the galaxy from their dark embrace for all time.
What is this path? What is the vision shown to Yoda? The answer, is for you to watch the entire Original Trilogy of Star Wars.