move music to iPhone 6S from macbook air

Are you asking about Apple Music subscription tracks or music you've purchased that resides in iTunes on the Macbook?

If its Apple Music then you don't sync AM tracks from iTunes to iOS devices. On your iPhone you would use the download icon (cloud with down arrow) to download songs you want on the iPhone for offline listening. You can download whole albums in one step or if you have a large number of tracks you want offline you can also add them all to a playlist and then select download of the whole playlist.