An inverter converts DC to AC, typically from a 12V or 24V car battery to 230V ac i.e. regular house mains voltage. Especially useful for caravans, boats and emergency stand by power.
To allow for start up power surges etc. our general advice is to allow a x3 rating factor, i.e. if you have a 500W ac product/device we would advise using a 1500W inverter.