A Karmasphere Plugin is a piece of software that you can install into a 3rd party product which makes it capable of making Karmasphere reputation queries. In other words, it turns a product into a Karmsphere Client.
Karmasphere makes a number of plugins available that give rich access to Karmasphere services. Note that any product capable of using a DNSBL and/or DNSWL can also make use of Karmasphere services.