This is all the necessary hardware to join two rectangular bases or triangular bases by stacking them. The bases have predrilled holes in them that fit these spacers.
The InvenSense MPU-6050 sensor contains an accelerometer and a gyroscope in a single chip. It is very accurate, this sensor is used in self balancing robots, drones, smartphones, etc. The sensor uses the I2C-bus to interface with microcontroller.