P0754 Code on 2002 Altima 2.5S

Can anyone tell me exactly what the P0754 code tells you on a 2002 Altima 2.5S? I know the lists say "P0754 Shift Solenoid A Intermittent", but I am curious as to what this means- what the part is, what it does, what could cause the code....? It's an Automatic.