The status bar at the bottom of the each file display is primarily used to display basic statistics about the current folder. Like most things in Opus, it's fully configurable.
This is an example of the default status bar. The information presented (from left to right) is:
You can use the File Displays / Status Bar page in Preferences to configure the information displayed in the status bar. Using the various available status bar codes it is possible to configure some quite complicated status bar displays, including detailed information about selected files, various bar charts and more.
The options on that Preferences page also let you change the status bar to appear at the bottom of the Lister instead of the file display (so, for example, you can have one single file display at the bottom of the window that applies only to the active file display instead of each display having its own).
Some status bar elements will show you tooltips with more information when you hover over them: