What does reversible mean?

Reversible is a term used for certain reactions that never reach a final stage. So imagine we have A and B reacting together to form C, and we say that this reaction is reversible. So at a point in time, in the container where we mixed the two reactants in order to create C, we have all three componenents - that is A, B and C. How much of each component we have at that time depends on the kind of reversible reaction. If a reaction is irreversible and goes to completion then at that point when the reaction is complete we would only have C in the container. However, for reversible reactions, C can go back to being A and B on its own and that is why the reaction is reversible and in the container all three components can be found at a time point.