Enabling vocal tonic note uses a more detailed vocal HUD in game that was introduced in The Beatles: Rock Band to make harmony parts easier to read
Without it, the vocal HUD uses generic lines with no pitch information

Very few customs and RBN songs use it. It is entirely optional
Do not use unless you know how to do it

The tonic note represents what musical key the song is in or what the 'home note' is for singers
It is a value from 0 to 11, counting in half-steps up from C

Song tonality lets you change the game from major keys to minor keys

DO NOT CHANGE SONG TONALITY UNLESS YOU ARE ABSOLUTELY SURE YOU KNOW WHAT YOU ARE DOING...

Many thanks to OrangeHarrison for this information