The state function entropy measures the increase (or decrease) in "disorder" of a system when it undergoes a change, such as a chemical reaction. Generally, reactions such as gas-phase dissociations proceed with a positive entropy change, since several fragments are created from an initial molecule. (Dissociations in a solvent are not easy to predict, however, as you will see as you work some examples).
Is a positive entropy change enough to ensure that a reaction will proceed? No. Entropy enters into one of the terms in the expression for the free energy, which determines spontaneity.
The table below shows the entropy changes associated with a number of reactions.