Bar Tracking refers to the vertical line that moves over the chart as you drag the mouse pointer.

Bar Tracking helps you identify which bar or tick in the chart corresponds to the price data that displays in the Data View window (if visible). The vertical bar shows the date of a trade or time increment in both the Data View window and on the time scale at the bottom of the chart. Moving the bar by one position in a Tick chart shows the price values for each trade, while moving the vertical bar in a Daily chart shows the price values for each day.

Bar Tracking, by default, is turned ON when a chart window is opened (this setting is found in the Settings menu on the Chart Toolbar).