This prophecy predicts a total and absolute financial collapse of the entire world economic system that will occur shortly after the start of the Tribulation when the first of the seal judgments are released.  The actual event in question, as spoken of in Revelation 6:6, namely the Black Horse, the third of four horsemen, one of the 21 judgments of Revelation, is set to begin shortly after the rapture, and may be initiated by the same said event.  This will in turn create great famine, pestilence and other things far worse.  Due to this, food is expected to become incredibly scarce and money and most forms of traditional barter and trade will be worthless, including gold and sliver.  It also appears to be one of several events that make up the Black Horse judgment.  AKA, the 3rd seal.

This collapse will literally create a scarcity of everything, even though food will be the primary item in short supply, either literally through a simple lack of sufficient food production and supply, a loss of food through drought, famine, disaster, plagues, etc, or possibly through governmental or economic forces that greatly limit or restrict the amount of food available to anybody at any one time.  The statement in Rev 6:5-6 seems to indicate that the latter is more likely to be true, and given what the governments of the world either already have, or are planning to do, namely clamping down on or extremely limiting access to food to everyone but a select few (namely the super rich controlling the world) this idea is further reinforced.