You know how it is. You're sitting in your bed, it's late at night, you listen to a friend's track. You think it's good -- so good it gives you goosebumps. So that's what you tweet.
Buuuuuut just in case something thinks the goosebumps are because you're totally gay for your friend, you tag it with "no homo", which should cover that assumption just fine.
Or maybe you don't do that, because that's really not okay.

Again, that's an actual tweet from Tyler Seguin, and no, unfortunately, he isn't a member of The Lonely Island.
For the uninitiated (you lucky people, you), the phrase "No Homo" has been around since the early 90s, mainly in street slang and hip hop. It is, as you can imagine, tacked onto the end of sentences to quickly establish that no homosexual intent was meant by whatever was expressed.
It implies that whatever you said in advance of "no homo" is somehow inherently a gay thing to say (seriously, goosebumps?), and that there's something wrong with that. The phrase is rooted in creating a needless distinction between the homosexual (icky!) and the heterosexual (acceptable!). It's a jittery, instant disclaimer that that exposes a childish fear of being seen as a closeted homosexual. Unless you're espousing your preference for 1% milk, "no homo", it's a little homophobic.