The strength of the current in the electrical circuit is measured using an ammeter.
There is a rotor with a coil wound inside the ammeter. When the current flows through the ammeter, the coil and the rotor will have the properties of a magnet. The higher the current flow, the stronger the magnetic field of the coil and the rotor. The needle is stopped at the balance point where the elastic force of the spring and the magnetic force.
A (very low) resistance is connected between the ammeter terminals. Most of the current entering the ammeter passes through this resistance and immediately exits the ammeter.
This will lower the total resistance of the ammeter and minimize the ammeter's impact on the circuit.