Symbolism, Imagery, Allegory
The image of the goddess (or semi-divine character) Fortuna and her rota fortunae (that's "wheel of fortune" to us) is a traditional literary symbol from classical literature. From rhetoricians like Cicero, Tacitus and Pacuvius, medieval writers got the idea that Fortune is a flaky, insane, and blind force that pays no heed to things like worthiness, unworthiness, good family or bad family, kindness or evil. In the early days (we're talking BCE here), she's pictured as a goddess standing on a rolling rock. Whichever way the rock rolled, that's the way she fell. And as Fortune falls, so do the fates of men and nations.
The extremely unfortunate 6th century Roman consul and philosopher Boethius is largely responsible for the Christian version of Fortune's Wheel as it was understood in medieval Europe. Boethius' work, written in prison awaiting execution) The Consolation of Philosophy (which is fantastic, BTW) has Lady Philosophy explaining to him that bad fortune can't be helped: fortune is random, her wheel always in motion, raising people up and letting them down like an existential Ferris Wheel. But in the Christian view, Fortune's not an independent goddess; she's in the service of God. Only God knows what's going to happen next, because he holds the reins of Fortune.
Boccaccio uses this image of Fortune freely in his works, especially on Day Two of The Decameron, where "changes of fortune" is the assigned topic. Although the brigata often acknowledge God as the author of some of the twists and turns of fortune, some of the stories can leave an impression more of a general randomness and unpredictability in human affairs. People are always being destroyed then saved; or saved and destroyed; finding then losing (or losing then finding) love or wealth. In many of the stories, the fortunes of the characters change so often that it's impossible to know how they'll end until they end. And even then, things are still up for grabs. Some of the plot twists are so ridiculous and profane that Boccaccio almost seems to be mocking the idea of God as the force behind them.
We have to remember that Fortune changes constantly—we can ride the wheel up and figuratively be on top of the world. But what goes up must come down, and we can find ourselves crushed like karmic roadkill at the bottom of the rota fortunae in the blink of an eye. After all, everything in life is only for now.