The MPR121 capacitive keypad uses the MPR121 and 12 touch sensitive pads to give you a simple 'keypad' with an I2C output. These keypads function by measuring the capacitance of twelve electrode points. When an object comes close to the electrode connector, the measured capacitance changes. This signals the MPR121 that something has touched a ‘button’.
The board also has four mounting holes allowing it to be used as an input system in place of traditional buttons. This keypad outputs 3.3V logic. If you are using a board which only accepts 5V logic, you will run into a problem. You will want to use a logic level converter, or a board which accepts 3.3V logic.