Q: I don't understand "Add to up next". Just want to play playlist in order.

Hi  jmcincinnati,

Thank you for using Apple Support Communities. Based on what you stated, it seems you want to get more information on what Up Next does. I would recommend you read this article, it may be able to help troubleshoot the issue. 

iTunes 11 for Mac: Ways to play songs

While listening to music, you can use Up Next to view and change which songs play next. For example, if you’re listening to a playlist, you can use Up Next to switch to an album after the currently playing song. When the album finishes, iTunes resumes playing the playlist.

Using the new Music app - Apple Support

Tap    to show your Up Next queue, which shows you all of the songs that you’ve selected for playback.

Cheers