How does an auto bidirectional voltage translator work?
PP (push-pull) means that the output provides current in both directions (sinking and sourcing).  A "1" on the output will provide a positive drive voltage at the logic level, and a "0" will pull the output down near ground.  This will not work with some bus circuitry such as I2C or SMBus because you cannot tie two push-pull outputs together.  If you do, and one is high and the other low, they will try to drive each other resulting in high current and an undefined output.

OD (Open Drain) outputs only sink current.  When the output is "0," the line is pulled down near ground, but when the output is "1," the line is high impedance, as if it was disconnected from the output circuit.  Therefore, you can tie many of these types of outputs together with a "pull-up" resistor (tied to the logic voltage) to pull the logic level up to the logic "1" voltage when all of the outputs on the bus are at the "1" state.  Since the outputs can only pull down, any of the outputs on the bus can safely pull the line to a logic "0" state when the other outputs are still in the "1" state.  In this manner, multiple outputs can "take turns" communicating on the same bus line.