Display masks alter the decimal place that prices are shown in. For example, a price could be represented as 1000.55555, and adjusting the display mask to show two zeros past the decimal would show the price as 1000.55. 

To change the display mask of a chart, right-click on the chart and select "Edit Instrument". 

Go to the "Advanced" tab. Enter the decimal as a # to the decimal place desired.

Once finished press "OK".