Well, it transforms the current to a higher or lower current (depends which is used as primary or secondary side) , but the voltage likewise in the opposite way. However, this requires AC currents, positive and negative currents to work. You can’t transform DC currents with it. And microcontroller analog inputs can only do something with positive voltages. So it won’t work. Even worse, the negative side of the wave could kill it.
If you want to measure dc currents then you could measure the voltage across a shunt resistor with an instrumentation amplifier or more basic an opamp, and amplify/convert that to the desired ADC input voltage.
Now it starts to get important you tell us exactly where these currents are comming from. Before you attempt to connect a transformer core between a mains-appliance and your AVR/arduino microcontroller. And set the house on fire.