Positive polarity, negative polarity, reversible polarity; why is this important when I purchase a supply?

DC sources are polarity specific. Using earth ground as a reference point, the output of a DC supply can be "X" number of volts above ground (positive polarity) or "X" number of volts below ground (negative polarity). Another way of explaining this, is as a positive supply can source (provide) current, while a negative supply can sink (accept) current. Applications that require DC high voltage sources are polarity specific, so the polarity required must be specified at the time of order.