Open drain Port Configuration
The simplified view of a digital "output" is simply two transistors, one connected to the ground, one connected to the circuit input-output power supply (Vio).

When and output is configured in "push-pull" mode, outputting a '0' connects the output pin to the ground, a '1' connects it to Vio.
In "open drain" mode, the "higher" transistor is always disabled: '0' still connects the output pin to the ground, but outputting a '1' does not connect the pin to Vio, and it's left "floating".

Configuring a pin as push-pull or open drain is a choice you make during a circuit design, depending on what you want to achieve.