Unlicensed music is music you don't have a license to use. It's pretty self-explanatory. Specifically, what Newgrounds doesn't want you using is music owned by big record labels.
As for whether or not certain music counts as "licensed", when it comes to big-label music, you'll have to pay for a license to use it, which is usually very expensive (though there is the SLIGHT possibility that they're nice enough to let you use it for free, but then they'll probably want you to use it non-commercially). Music listed under creative commons on the other hand, IS already licensed, because creative commons IS a license, granting you rights to use the music under certain conditions, determined by the artists, though most artists usually just want you to credit them if you use it. Every CC license is different, so pay close attention to which license it is, especially on freesound. I downloaded some stuff on freesound, thinking it was all 100% royalty free, only to realize later that some of it was non-commercial and other stuff.
Kevin MacLeod's music is all licensed under creative commons - attribution, so all you have to do to use it is credit him. Newgrounds audio portal music is all licensed under creative commons, but they're all different licenses, so keep an eye out for which one, though iirc, newgrounds audio portal music is free to use automatically for web-based games and movies.
Also, public domain stuff is 100% free to use, period. It's not copyrighted, because it either fell into the public domain for being too old, or its creator decided to make it public domain, and so you don't need a license to use.
When it comes down to it though, if you're ever unsure if you can use a certain piece of music, find the artist and ask permission.
