We recently received a letter stating our Carte de Sejours (residence cards) were approved and ready for pick-up at our local prefecture and that we must bring 106 euros in timbres fiscaux.
Simply put, timbres fiscaux are stamps that act as currency and look practically identical to postage stamps. They are sold in various amounts with the value marked on them. They can be purchased at the Tabac Shops which are numerous all over France. No ID or documents are needed to purchase the stamps and you can purchase them with whatever means of payment the Tabac Shop accepts- usually credit cards, cash or even checks. When you go to the prefecture or whatever agency to pay with the stamps, the clerk will stick them on the back of the signed document- (for example, the document I was required to sign in order to pick up my residence card.)
For examples of situations in which you can use the electronic timbres fiscaux, view this French government webpage: https://timbres.impots.gouv.fr/