1. what does it mean when you dream of HH dalai lama? i read somewhere that it's not really a "dream"...?
Who knows? If you attended a teaching with HH, it's to be expected. If you saw him on TV, is not unlikely, if you've been reading about Dharma, the same. If it came out of nowhere, it's interesting.
Anyway, I hope it was a pleasant dream, but don't get hung up on the experience. Write it down if you think it's important.
2. what is the naga realm?
It's the samsaric experience of naga beings, meaning their conventional reality. Our human experience of samsara is the human realm. So in a way the naga realm is their "dream like reality" as our human realm is our "dream like reality". Both make contact cyclically. It's important not to take the interaction with nagas lightly as they are easy to provocate and then can cause some disagreeable consequences in one's life.