The level of the MPU6050's AD0 pin input determines its I2C address:
AD0 = 0 (GND) : I2C Address = 0x68
AD0 = 1 (3.3V) : I2C Address = 0x69
(Note: make sure it's pulled to 3.3V, since the MPU6050 isn't 5V tolerant).
This allows two MPU6050's to be used simultaneously on the same bus.