This is one of the types of indicators (see) that analyses the market. These indicators appear in a separate window, and are usually depicted as bars - histograms. If you look at the list in the "Navigator" window, you will see that next to the name of some indicators is the word Oscillator. However, there are oscillators that are unmarked, such as MACD, which stands for Moving Average Convergence / Divergence (convergence / divergence of moving averages). This indicator works well when the market is moving sideways, without any particular direction, flat (see). That's when the oscillator helps, as it determines when the price is finally changing direction and going either up or down.