The output of an Arduino pin - when set by the program to be an output - is in the form of a "latch"; when the program tells it to be a logical "1" or HIGH, then it switches to 5V and stays there until another program instruction tells it to be a logical "0" or LOW, at which point it switches to 0V (and can "sink" the rated amount of current from something to which you connect it) and stays that way and so on.
Do read all through the tutorials and information available on the Arduino site as I cited before.