Shunt is the resistance that is connected in a circuit with Galvanometer to change its effective resistance.
Voltmeter - as voltmeter measures voltage across two points, we need no current to pass in to the voltmeter (ideally). So very high resistance R1 is connected in series with the galvanometer.
Ammeter - the galvano meter is connected in series in the circuit - because the ammeter must measure the current in that circuit. Hence to minimize the change in the current actually flowing, we use very small resistance as the shunt.
shunt resistance S.
Actual resistance across two points : R, Voltage V to be measured, current = I.
After connecting, effective resistance = RS / (R+S)