A system is in physical equilibrium when the amount of matter in each of its phases stops changing with time, with no perceptible chemical change occurring -- only a physical one. This kind of equilibrium is possible only in a closed system held at a given, fixed temperature: an open system would let matter escape (a melting ice cube left in the open air never reaches a solid-liquid equilibrium, it just keeps melting), and a changing temperature would keep shifting where the balance point sits.

Three classic phase-change equilibria illustrate the idea. Solid-liquid equilibrium: ice and water together in a closed flask at 273 K and 1 atm reach a state where the rate of melting equals the rate of freezing, $H_2O(s)\rightleftharpoons H_2O(l)$ -- the temperature at which this balance holds is the substance's melting (or freezing) point. Liquid-vapour equilibrium: liquid water and water vapour in a closed vessel at 373 K and 1 atm reach a state where the rate of evaporation equals the rate of condensation, $H_2O(l)\rightleftharpoons H_2O(g)$ -- the temperature at which this holds is the boiling (or condensation) point. Solid-vapour equilibrium: solid iodine sealed in a transparent vessel sublimes until a steady violet vapour colour is reached, $I_2(s)\rightleftharpoons I_2(g)$ (similarly for subliming camphor or ammonium chloride). …