Up Next is a new way in iTunes 11 to choose what songs to play and in what order, sort of like the old iTunes DJ feature. Here's the simple version of how it works: Play something from your iTunes Library, and Up Next will add subsequent tracks to its list to play afterward, which you can view by clicking on the three-lined icon at the top of the iTunes window.
But there's a lot of stuff to know once you understand what Up Next is doing. First, if you picked an album to play, you'll get the other tracks from the album added.
See how it lists the album name at the top of the Up Next window? That's how you can tell what it's gonna play.
If you've chosen an artist, Up Next will include all albums from that artist.
Simple, right? But now listen to this. When you choose an individual track from your Songs list, iTunes'll add additional music to Up Next based on how you've sorted things. So note that in my screenshot below, I've clicked on the "Name" column header to alphabetize my songs.
If I start playing one, the subsequent songs get added to Up Next in the same order.
So you could play your entire iTunes Library sorted by album this way if you wanted to. Or, heck, you could sort by rating, or number of plays, or whatever. That'd be a weird way to listen to your music, but I ain't judging, man.
Removing Music and Changing the Order
To remove something from your list (and keep it from being played), hover over the song in the Up Next window and click the "X" next to its name, or Command-click to select a bunch of songs and press the Delete key to remove them all at once.
Click the Clear button at the top-right of that window to get rid of everything that's currently set to play.
To switch the order, just pick up a song (or Command-click to select multiple songs, then pick those up), and then drag 'em around in the Up Next window.
Adding Songs to Up Next
If you want to add something, you have about 49,765 ways to do so. For starters, you can drag a song, album, artist, or playlist up to the player at the top of the iTunes window and drop it, and it'll get added to the end of your Up Next list.
You can right- or Control-click on an item and choose Play Next from the contextual menu, or select Add to Up Next, which again will stick it at the end of your list.
Those same options are also available if you click on the right-pointing arrow that appears when you have something selected or when you hover over a song.
If you just double-click on an item out of your library when you've already got stuff in Up Next, iTunes will ask you if you want to clear what you already had.
Use It with the MiniPlayer!
Finally, Up Next also works with iTunes' MiniPlayer. You can invoke the MiniPlayer by either clicking its new icon in the upper-right of the iTunes window or by hitting Option-Command-M.
And then you can just click the same Up Next button to view what's coming up right from your MiniPlayer.
Holy crap, that's a lot of information. And all about one teeny new feature of iTunes 11! You guys, when we get to iTunes 12, you may have to download a 50-page PDF to read about how to play a song. I apologize in advance.