The meaning of LOW also has a different meaning depending on whether a pin is set to INPUT or OUTPUT. When a pin is configured as an INPUT with pinMode(), and read with digitalRead(), the Z-Uno will report LOW if a voltage less than 2 volts is present at the pin (3.3V boards).
When a pin is configured to OUTPUT with pinMode(), and set to LOW with digitalWrite(), the pin is at 0 volts. In this state it can sink current, e.g. light an LED that is connected through a series resistor to +3.3 volts.