"Mola" means "mill", "millstone", and by extension, "flour". "Salsa" is the matching form of the adjective "salsus" meaning "salted". So mola salsa means "salted flour".
History of the mola salsa
It was first introduced into Rome by the Sabine king, Numa, as a propitiation to the gods. It was looked upon as a bloodless sacrifice and performed at religious weddings when a pig was not offered.
It was Numa who first established the custom of offering grain to the gods, and of propitiating with the salted cake. Ovid, in his Fasti, says the atoning sacrifices (piamina) were called Februa, and the parched spelt, with the grain of salt, was also called Februa, and that it also gave its name to the second month.
These cakes were sprinkled over the head of the animal sacrificed by crumbling them into tiny pieces. The Latin word, "immolare," originally from "mola," meant to sprinkle the victim for sacrifice, hence we get the English word to "immolate." The mola salsa was made from spelt, a sort of rough wheat still grown in the more mountainous districts of Europe. Some sources say that on February 15th, the three elder Vestal Virgins gathered the first ears of each harvest for use in making their sacred cake. Other sources claim it was done from May 7th through May 15th.
Making mola salsa
Salt was used for purification, and also for making mola salsa, a purified cake made with a mixture of flour, water and salt. To make mola salsa with salt from the salinum, mix a small portion of spelt flour with a small amount of water until it becomes a paste. Add some salt and 'knead' it a bit with your fingers. Flatten it into small, round wafer-like cakes, the thinner the better. The cakes can be burned in the turibulum as an offering to the gods.
According to Lucia Modia Lupa, the preparation of Mola Salsa is done by grounding with mortar and pestle the spelt and then roasting it. A bit of salt is added and you can use it either in its powdered form or add a tiny bit of water and bake them into cakes depending on how you are using it (for offerings bake into cakes obviously). It really is best though stored in its powdered form, baking a small amount once a week or however often you may need cakes.
Today, the closest example of this type of "bread" is the large, round flat wafer used by the Catholic Church in the exposition of the Blessed Sacrament. It is, in fact, the sacred cake of the Vestals - the mola salsa.
Uses of mola salsa
The sprinkling of mola salsa on sacrificial victims gives us the word immolatus. According to Rich:
- Proprement, saupoudré de farine (mola salsa) ; se dit d'une victime destinée au sacrifice : c'était une des cérémonies habituelles avant de l'égorger (Cato, ap. Serv. ad Virg. Aen. X, 541) ; de là ce mot a été employé dans le sens moins spécial d'immolé ou égorgé en sacrifice (Hor. Od. IV, 11, 7). 
- ↑ Rich, Anthony. 1883. Dictionnaire des Antiquites Romaines et Grecques, (3e ed.)