それな is another way to say 「そうですね」, which is a phrase to express agreement, as in "I agree," "I think so too," "Yeah, that's right!", "You can say that again," and "I know what you're saying." (A more casual version of 「そうですね」 is 「だよね」.
At this time, それな is a younger person's expression. One thing worth noting is the intonation; people tell me that it should be spoken with a rising tone. Below are some examples.
Apparently people sing about it:
AVEX won't let you see the video on this blog, but the URL is:
https://www.youtube.com/watch?v=Pgvb6t2oLqg
and laugh about it
------------------------------------------------------------------------------------------------------
And this was kind of interesting I thought, partly because she slides back and forth so effortlessly between Korean and Japanese. I don't know much of what she's saying, but it's somewhat heartening to me, given recent tensions between South Korea and Japan. Nice that there are people who move toward bridging things (which is what I think she's doing, but since I don't speak Korean, I can't be sure).
This is Jpop in its high-pitched revelry: