12-0-12 500mA Transformer
It is a 12-0-12 center tapped step-down transformer. Meaning it can take standard AC mains voltage of 220V AC and step it down to 12V AC and 24V AC. Either of this can be used as output voltage. The voltage across the yellow and blue(any one) wire will be 12V AC and the voltage across both the blue wire will be 24V AC. The maximum output current the transformer could deliver is 500 mA.