In the directory you've chosen, select the files by clicking their checkboxes
(the "Sel" column contains the selection number following the order you've done it)
In the "Expression" combo, enter a renaming expression,
validate it with the "Enter" key.
In this case, "%n" corresponds to the selection number and "%e" to the file name extension.
The "Future name" is immediately computed.
Click the button with the "Play" shape, the renaming is done.
The "Undo" button has become enabled.