Bells and whistles are nice but nonessential add-on features. A new car doesn't need a lot of bells and whistles to run, but extras like a built-in stereo and cruise control sure are nice.
The idiom bells and whistles may have come from early trains with fancy, specialized bells and whistles, or it may have come from fairground mechanical organs that were literally outfitted with added-on bells and whistles to grab attention. Today, the phrase describes the premium or flashy features of a product, such as advanced cameras on smartphones or sunroofs on cars. These things aren't necessary for these items to work, but they may attract more buyers willing to pay extra.