French revolutionary
assignats were backed by the value of lands confiscated from the old state-sponsored church. However, there was only so much land to be confiscated, and the land itself quickly fell in price because nobody could afford to buy it. But the printing presses kept running; assignats were one of the earliest examples of what we now call "hyperinflation" - when inflation gets so out of control, the printers can barely keep up with the increasing face values and new note issues required. A situation very similar happened in the US with the Continental Dollar, except in France, it was even worse.
The "sol" (in French Canada, it was called the "sou") was a secondary money of account in France, immediately prior to the revolution. There were 12 deniers to a sol and 26 sols to a livre. A 1 sol coin was copper or brass, and worth about the equivalent of a US cent or British halfpenny. By January 1792, the date on your note, the inflation was well and truly underway, though not quite in the "hyper" phase. So a "15 sols" assignat wouldn't have had a very high face value to start with; it would have quickly become worthless as the hyperinflation progressed.
