"Nun" and "mal" are emphatical markers that have no meaning of their own here. The sentence means "That's the way it is", "That's how it is", "This is just how it is". Without emphatical markers the German sentence could be "So ist das" or "Das ist so" (This is so; It is this way).
https://askaboutgerman.wordpress.com/2013/03/07/difference-jetzt-nun-german/ the first comment on this post explains the usage of 'nun mal'
I tried "It is what it is" and it was not accepted, but it should be. I have reported it. But for non-native English speakers, here's the wikipedia article (http://en.wikipedia.org/wiki/It_Is_What_It_Is). It's perhaps more colloquial than the official answer, but no less correct and something you would hear someone actually say way more often than the official answer.
The two English phrases have subtly different meanings. "That's just how it is" implies that it's a fact, regardless of argument but doesn't necessarily mean it's bad, whereas "It is what it is" implies that something is not (very) good, but one must accept it (it's also so overused that it's become trite). The first is accompanied with a shrug, the second with a disparaging shake of the head, or maybe a frustrated eyeroll. I really don't know how the two line up with "So ist das nun mal" however...
It seems to me that "That's just how it is" and "It is what it is" share in common that the speaker assumes that the hearer sees the fact as bad, but that the speaker also sees the fact as something that must be accepted (I don't think that only "It is what it is" has this connotation). I'm not 100% sure about the difference in connotation between the two, but perhaps the first one suggests that the speaker accepts the fact (hence it sounds more like something that a parent may say to a child e.g. explaining mortality), whereas the second may suggest that even the speaker is struggling with accepting it (hence more likely said among peers). Though whatever difference there may be, it's pretty subtle, and I have no idea which (if either) that the German expression is closer to.