Why am I getting an authorization error when syncing my iPhone music?

I can't sync my iphone because I keep getting this error. Some items including" the name of the song" were not copied to the iphone because you are not authorized for on this Mac. I have no idea what to do. Why can't Apple fix this themselves? This is so frustrating for people like me that don't understand TEC talk. Please someone help. I've been an Appple customer for years and I'm now hating Apple.

[Re-Titled by Moderator]