No such thing as a stupid question!
Sponsons aka "the inflatable bit" of a RIB are a fundamental part of a RIBS design. They provide bouyancy and are usually split into 5-7 seperate compartments for safety in event of being holed. When a RIB us at rest the sponsons provide stability - In some RIBS they rest on the water giving max stability, in other more high performance models they are slightly clear of the water which means the boat tips over before they come in contact with water and stop it tipping any further.
And finally the really good thing about Sponsons and the bit that makes a RIB much more comfortable and seaworthy than the equivalent sized "hard" boat is that when under way, particularly in rough water the tubes provide a cushioning effect producing a much softer ride!
A good book, although slightly dated now is "Inflatables" by Dag Pike. Gives all the essential background on RIBS and pure inflatables together with driving techniques etc etc.
A recommendation would be to get get some formal training by a qualified powerboat instructor in a RIB. I believe you are in Ireland - try contacting Powerboat who posts here, he should be able to help you out!
Hope this is of help. Enjoy Ribbing!