To oversimplify, medieval/renaissance monarchs generally had three major sources of revenue:
- Their own lands, which generated revenue like any other feudal holding
- Sale of offices, monopolies, charters, patents, etc
- Taxes
Taxes differed significantly from the other two categories, since taxes generally needed to be authorized by the realm's legislative assembly (Parliament in England, the Estates General in France, the Cortes in Castille, etc). In general, the more the government depended on new or renewed tax legislation, the more powerful the legislature was relative to the king.
Late
Ancien Regime France is often described as an absolute monarchy, but as @
Basileus_Komnenos pointed out, that's a misleading oversimplification. The Kings of France were a lot more absolute than their English counterparts, and one of the big reasons for that was that England's kings needed quite a bit of tax revenue to cover their expenses and England's Parliament made a point of only authorizing taxes for a few years at a time so the king had to keep going back to Parliament for more money. France's Estates General, on the other hand, had authorized several permanent in the 1400s. Most notably the
Taille, a tax on non-noble lands (*) with the interesting feature that it wasn't assessed at a fixed rate: the King decided each year how much money he needed, and that amount was apportioned across provinces, each of which in turn apportioned it among landowners. So the king could raise and lower the main tax at will (subject to his practical ability to collect the tax, of course), but he couldn't change its structure without reconvening the Estates General.
As it happened, the Kings of France had enough revenues from the Taille and other permanent taxes in addition to their own feudal incomes that they didn't bother calling the Estates General into session at all for the 175 year period from 1614 until 1789. Some kings (most notably Louis XIV) also got away with imposing additional taxes on their own authority. I'm a little fuzzy on the details of how, but it sounds like it boiled down to the King decreeing the tax and the appeals courts that could have declared the taxes illegal (the Parlements) being abolished/suppressed by the King at some times, and being bribed, intimidated, or persuaded to back the King's policies at other times.
Louis XVI had allowed himself to be persuaded to reestablish the Parlements, which were dead-set against new Royal taxes, and he didn't feel he was in a position to abolish them again. And he needed more money to pay France's debts than he could reasonably raise with the Taille and other existing taxes, so that left reconvening the Estates General as a means of restructuring France's tax system.
(*) This is subtly different from "lands owned by non-nobles". If a noble sold his land to a commoner, the land would remain "noble land" exempt from the Taille.