Nearly all plug-ins have some sort of parameters that control their behaviour, the radius of a circle drawer, the frequencies to filter out of an audio signal, the colour of a lens flare and so on.
Seeing as hosts already provide for the general management of their own native parameters (eg: persistance, interface, animation etc...), it would make no sense to force plug-ins to do this all themselves.
The OFX Parameters Suite is the means by which parameters are defined and used by the plug-in but maintained by the host. It is defined in the ofxParam.h header file.
Note that the entire state of the plugin is encoded in the value of its parameter set. If you need to persist some sort of private data, you must do so by setting param values in the effects. The Sync Private Data Action is an action that tells you when to flush any values that need persisting out to the effects param set. You can reconstruct your private data during the Create Instance Action.