When checked the application uses the Breeze icon theme.
When unchecked the application uses the system icon theme. Doesn't always work as expected.

When checked this option makes the Qt Quick Controls (buttons, checkboxes etc.) use the Breeze style.
When unchecked the controls should look according to the system theme. Doesn't always work as expected.
If the control is disabled (can't be checked/unchecked) it means the Breeze style is not installed.