These options are for the radio item "Action for dragging of tabs" => "Select tabs".
In the mode, when you press the mouse's button on a tab and drag over some tabs and release the button,
they are selected like cells in the Microsoft Excel.
After the operation, you have to choose commands for selected tabs from a menu.
If you enable "Open popup menu automatically", the menu is automatically shown just after you release the mouse's button.

With unchecked "Clear tab selection after the popup menu is closed",
selected tabs are keep themselvs selected after you close the popup menu by clicking outside of the menu.
To unselect those tabs, you have to do Ctrl-click on them.
If you check the checkbox, selected tabs lose their selection just after you close the menu.