There are only a finite number of I/O pins available on Arduino boards, and sometimes the pins used on a shield conflict with other shields or even the Arduino board itself. In the case of the USBDroid, digital pins D9 through D13 are used for the onboard USB-host functionality that allows it to operate as a peripheral for an Android device such as a tablet or phone. However, the LCD & Keypad Shield also wants to use D9 so if you plug them together you need to reassign one of the pins to avoid a conflict.
It's really not hard once you know what needs to be done, and we've explained the options in a new tutorial:
