A stock's price does not move in a completely continuous fashion. It moves in discreetdiscrete steps depending on who is buying/selling at given prices.
I'm guessing that by opening bell the price for buying/selling a particular stock has changed based on information obtained overnight.
A company's stock closes at $40. Overnight, news breaks that the company's top selling product has a massive defect. The next morning the market opens. Are there any buyers of the stock at $40? Probably not. The first trade of the stock takes place at $30 and is thus, not the same as the previous day's close.