15A 250V Extension Cord, but the voltage rating in my country is 220... What does that mean?
It means you're fine. As long as the rating of the cord (250V) exceeds the actual voltage (220V), there's nothing to worry about.

The ratings of the extension cord are maximum values that should never be exceeded. It will work fine for any voltage less than 250V and for any load current that is less than 15A.