This menu item selects all keywords that are very similar to an already selected keyword. To use this menu item, first select a keyword that is part of a tower in the 3DView Area. Then click this menu item to select very similar keywords (normally those in the same tower). If the tower contains keywords that should be pruned out, then click the Delete/UnDelete Keywords to remove the tower.
When a keyword is selected, a checkmark appears in front of the keyword, and the box in the 3D view area representing the keyword turns to a color chosen by Tools | Change Colors... (normally yellow).