[quote name='bazztard' timestamp='1491648571' post='3274544']
you've never heard this term used in relation to scale notes?
Bass players don't play chords in Stand By Me. They play chord tones, made up of the Root third and fifth of the major scale for major chords.
[/quote]
More correctly the "Tonic".. If E minor and G Major use the same notes, how can you tell the difference ? This is part of the OP's question.. It's the "home" note basically (it's actually a bit more complicated than this but it's a good place to start).. In stand by me's OP's case it';s pretty easy, it starts and ends on G Major... I've just played it on the piano with G Em C D G in the right hand and E C# A B E in the left hand and it sounds very dark (and weird) ..