It’s basically a galvanometer which is modified to measure current. A galvanometer will give full scale deflection with very samll current of few milliamperes or micro-amperes. So it can not be directly used to measure heavier currents.
To measure heavier currents a low resistence is connected parallel to galvanometer. Then it becomes ammeter. The low resistance so connected is called shunt. The shunt provides an alternate path to flow of current since its resistance is low most of the current passes through it and only a small fration of current passes through galvanometer. However the deflection in galvanometer will be proportional to the total current because if the total current is increased its fraction through galvanometer will also increase and there will be greater deflection.
By using shunt resistances of different suitable values , the ammeters of different ranges of current can be made by same galvanometer.
The ammeter is always connected in series in the circuit to measure current in that circuit.