Entropy is not a substance but a tendency. The Second Law of Thermodynamics states that the entropy of an isolated system never decreases. In plain language: things fall apart. Order dissolves into disorder. Concentrated energy disperses. Structures decay. The universe trends toward equilibrium — a state of maximum entropy in which nothing further can happen.
Entropy is the ultimate form of dissolution. It is not a force but a statistical inevitability. There are vastly more ways for a system to be disordered than ordered. A deck of cards has exactly one arrangement that is sorted by suit and rank; it has approximately 8 × 10^67 arrangements that are not. Shuffle the deck, and it will almost certainly become less ordered. The same logic applies to every physical system.
The dissolution of structure by entropy is not reversible at the macroscopic scale. You can reassemble a shattered vase, but the energy required to do so increases the total entropy of the universe by more than the reassembly decreases the entropy of the vase. The dissolution is permanent in the only sense that matters: it cannot be undone without making something else worse.
Entropy does not dissolve things quickly. It is patient. It works through intermediaries — rust, rot, erosion, fatigue. But it works without exception. Everything dissolves eventually, even if "eventually" means timescales that make the age of the universe look brief.
In Fiction
Isaac Asimov's The Last Question (1956) is the definitive fictional treatment of entropy. A supercomputer is asked, repeatedly across aeons, how to reverse the heat death of the universe. Its answer, delivered at the moment of final dissolution: "LET THERE BE LIGHT." The dissolution is reversed, but only by an act of creation so total that it is indistinguishable from divinity.
See also: The Heat Death of the Universe.