Thisis a blow-up of the last 2hours of the DOW. Look from left to right at the bottom. This represents the trades that occur within it's timeframe, i.e., each bar shows the total amount of trades executed at that time. See how they slowly start rising into the close and how they just shot upwards right in the last few moments of the session.
Those last few represent a few things. The momentum traders who take positions on the open and longs who have decided to sell. It also shows new positions being taken on the close. THese are also mom traders that want to sell into the next day. If it gaps up, like today then this is a profitable trade to make. However if it drops you have a decision to make…do you hold or wait or just take a loss.
Gap up or down means it opens well above or below the previous day's close- this is called the neckline.