What do you mean by a shunt?
An ammeter only carries a very minute amount of current, compared to the amount that it may be measuring.
A shunt is a resistance that is lower than the resistance of the meter movement itself, this shunt is placed in Parallel with the meter movement.
In this way, when connected to the circuit, the majority of current is passed, or Shunted, through the Shunt resistance.
Thereby ensuring that the meter only recieves a small proportion of the total circuit current and prevents the movement being damaged.
Hope this helps.