The Oxford Idioms Dictionary for Learners of English, 2nd ed. gives the following definition:
the thing (about/with something) is (spoken)
used to introduce an important fact, reason, or explanation:
○ I know you want to expand the business. The thing is, we haven't got the money to do that.
○ I'm really sorry I didn't call you. The thing is, I've been really busy lately.
In my own words, I'd say that this phrase optionally sets a topic of discussion (given with "about topic" or "with topic"), and then it announces to listeners that you're about to say something important about that topic.
Your example is a variation on this idiom:
The thing about Nickelback is, ...
This is the thing about Nickelback.
These two examples mean the same thing. They introduce a topic (Nickelback) and let the listener know that you're about to say something important about that topic.