If you pick a book off the floor and put it on a table, for example,
you're doing positive work on the book, because you supplied an upward
force and the book went up. If you pick the book up and place it gently
back on the floor again, though, you're doing negative work, because the
book is going down but you're exerting an upward force, acting against
gravity. If you move the book at constant speed horizontally, you don't
do any work on it, despite the fact that you have to exert an upward
force to counter-act gravity.