This menu item is available from the Clip Jobs menu that appears when you right-click on a clip in the Project Tree or from under the Project menu when a clip is selected in the Project Tree. It is used to create a clip which plays in reverse.
This feature became available in version 0.9.6 of Kdenlive.
When you select theoption from the menu, a new clip is created in the Project Tree. It has the same file name as the clip from which it was created, but with a .mlt extension appended. You can then add this clip to the timeline and when you play it, the video of the original source clip will played, but in reverse.