RAINS is the AE modular adaption of Mutable Instruments' Rings module. Rings was (as not in production anymore) a resonator.

When something resonates, by being hit, plucked etc. the material has frequencies that are naturally created; this is why acoustic instruments like guitars, drums, cymbals etc. have distinctive sounds; this can be subtle to wildly differing sounds for the same hit/note played. A resonator recreates this natural behavior enabling more "realistic" emulations of acoustic sounds or, much more fun, making a synthesised sound seem more natural to our ears. It can also create a whole other areas of sound for the synthesist to explore....

There are also some hidden modes; see the patch suggestion section for more information.

https://wiki.aemodular.com/pmwiki.php/AeManual/RAINS