HIGH | LOW

When reading or writing to a digital pin there are only two possible values a pin can take/be-set-to: HIGH and LOW. These are the same as true and false, as well as 1 and 0.

The meaning of HIGH (in reference to a pin) is somewhat different depending on whether a pin is set to an INPUT or OUTPUT. When a pin is configured as an INPUT with pinMode(), and read with digitalRead(), the Arduino (ATmega) will report HIGH if:

A pin may also be configured as an INPUT with pinMode(), and subsequently made HIGH with digitalWrite(). This will enable the internal 20K pullup resistors, which will pull up the input pin to a HIGH reading unless it is pulled LOW by external circuitry. This can be done alternatively by passing INPUT_PULLUP as argument to the pinMode() function, as explained in more detail in the section "Defining Digital Pins modes: INPUT, INPUT_PULLUP, and OUTPUT" further below.

When a pin is configured to OUTPUT with pinMode(), and set to HIGH with digitalWrite(), the pin is at:

In this state it can source current, e.g. light an LED that is connected through a series resistor to ground.

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 Arduino (ATmega) will report LOW if:

a voltage less than 1.5V is present at the pin (5V boards)

a voltage less than 1.0V (Approx) 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 (both 5V and 3.3V boards). In this state it can sink current, e.g. light an LED that is connected through a series resistor to +5 volts (or +3.3 volts).