Why should an ammeter have low resistance?

Hint: Ammeter is a device which is used to measure the amount of current passing through the circuit, and it is connected in series with the circuit. As stated, the ammeters have low resistance. We can use ohm's law .i.e $V=IR$ give a plausible reason.

Complete step-by-step answer:
We know that ammeter is used to measure the current in the circuit. We know that the SI unit of current is ampere, hence the name ammeter. The ammeter can measure small currents of the order pico meters to a few micrometres. However, the range of the ammeter can be increased if needed.
To measure the current, it is connected in series with the load. Since in series connection, current remains constant. We know from Ohms law that $V=IR$, or $I=\dfrac{V}{R}$, clearly more the resistance of the ammeter, less the current which flows through the ammeter.
Hence in order to measure the maximum current in the circuit, the ammeter must have low or negligible resistance.

Additional Information: The basic principle behind ammeter is the tangent galvanometer. Hans Christian Oersted in the year 1820, accidentally discovered that a bar magnet when placed near a current carrying conductor, gets deflected.
Various modifications are made to this galvanometer in the modern days. The other variations are, moving magnet ammeter, hot-wire ammeter and digital ammeter to name a few.

Note: An ideal ammeter is said to have zero resistance. It is used to measure current in the circuit. It is connected in series with the load, and in order to measure the maximum current in the circuit appropriately, it must have zero resistance. It follows the principle of moving-coil or tangent galvanometer.