To answer the exact question, 220 Volts RMS is the voltage of a supply of electric power which is twice as big as 110 Volts RMS.
RMS is an acronym for, 'Root Mean Square'. In essence, it's an overall average voltage rating which tells you the 'real work' which can be done by a supply of power, or, in other words, it is a truer rating of the overall power curve of a supply of voltage.
To determine an RMS voltage of a sine wave (as is used in AC mains power distribution), you measure its peak voltage and multiply it by .707, which will give you the RMS voltage rating.
So a 110 Volt RMS mains supply actually has a peak voltage of about 155.6 Volts and a 220 Volts RMS mains supply actually has a peak voltage of about 311.2 Volts.
Note: ordinary electrical test meters (multimeters) normally measure the RMS value, not the peak value, when they are used to measure an amount of AC voltage or current