It is the difference of the trade volume at bid vs offer. Depending on the configuration, it can track the accumulated difference over the course of the trading day.
Volume Delta Properties
To pull up the study property You can either right click in the chart and select properties from the menu or click the Chart Properties button
Simply Left click on the Value column to configure Volume Delta properties.
TIP: To add a multiples of the same study simply go to the add studies page select the study again then go to properties to configure each individual study.
Click here to return to the list of available Studies