Definition: Fish placed in the same aquarium to reassure or soothe other fish. Usually used with a potential breeding pair to encourage them to spawn by giving them a sense of security. They may also serve as a "target" for the displacement of aggressive behavior between a potential breeding pair.
Thats what dithers are supposed to be.. or they can be used so that aggressive fish will ignore each other.. Say if you want to keep C.A Cichlids together, youll put in silver dollars and tinfoils as dithers to keep the cichlids busy.
They sometimes keep shy fish active....something for an oscar to chase for example....or maybe just to show others that its alright to come out of hiding.
I have two gold belly barbs as dithers to help keep my Pbass from taking shots at each other or my oscar. But they also do a great job as a clean up crew, eating up all the left overs...
I've always used a bunch of zebra danios as dithers for all my tanks. They're hardy as hell, active, and cheap. If a few get eaten down the line...who cares.