The safety tag is for the well-being of the people and objects around you, and yourself.

Some general precautions for your Arduino:

• Don't under/over power your Arduino
• Don't pull more than 40 mA on most boards from one digital pin
• Don't apply more than 5V to any pin (except VIN)
• Don't change a circuit while the Arduino is powered

Some general precautions for you/the people around you:

• Don't apply mains to your Arduino.
• When powering high voltages with a relay, make sure to cover and seal any wires that might be connected to the high voltages.
• Arduino not to be used as a flotation device. Really, don't get your Arduino or any other part of your setup wet.