In Physics equilibrium could be when two or more forces balance each other, the body is in uniform motion or at rest.
In physics/thermodynamics, an equilibrium exists when two bodies at different temperatures come in contact and after heat transfer the two bodies reach the same final temperature.
In chemistry equilibrium exists in some reactions in which reactants produce products and the product compounds also combine to produce the reactants. So both ways the reactions occur. Usually these reactions occur in solutions and with ionic exchanges.
When the rates of production of product compounds are equal to the rates of production of reactants, then we say that the reactions are in equilibrium. That means the amount of products and the amount of reactant compounds are stable and constant over time.