Question:

What does it mean that when Moshiach comes, we will receive a "new Torah"? Isn't the Torah eternal?

Answer: 

The revelations will be completely beyond anything that we have learned until now.

One of the central prophecies of redemption is that "a new Torah will go forth from me." (Isaiah 51:4) Our sages explain that with the redemption, G-d will reveal a new Torah through King Moshiach.

This needs explanation: According to a halachic ruling in Maimonides, the Torah given to us at Sinai is eternal and will never be changed or replaced. This is according to the verse, "everything that I command you, you shall keep to do; do not add to it or take away from it." There is also a verse, "The revealed things are for us and our children forever, to carry out all the words of this Torah." This teaches us that all the commandments of the Torah were given to us forever. Therefore, we must understand the nature of this "New Torah," since nothing can ever replace or even supplement the Torah that we were already given at Sinai.

The Torah given to us at Sinai is infinite and cannot be fully apprehended by our intellect. Every generation of scholars tried their hand at interpreting Torah's mysteries, and constantly developed new insights and innovations in Torah's teachings. Regardless of the many interpretations that have already been brought forth, there are still deeper levels that have yet to be revealed.