The opening price is the price at which a security first trades when a market session begins. On many stock exchanges it is not simply the first trade to occur. It is set by an opening auction, in which orders collected before the start are matched together at the single price that allows the most shares to trade. The result is the first of the four values in open, high, low and close data, followed by the high price, the low price and the closing price.
The opening price often differs from the previous close, because news released overnight and trading in other time zones are absorbed all at once. The difference is called a gap. Trading just after the open tends to be heavy and volatile, with a wide bid-ask spread. Markets that trade around the clock, such as forex, have no true open, and data vendors fix one by convention.