Many chemical reactions are irreversable. They only proceed in one direction primarily because certain reactants are taken out of the system. The slightly soluble product will effectively remove ions of the salt from the system so that they can't become reactants themselves in a reversable process Formation of a gaseous substance that is allowed to escape by diffusion.
Many reactions are reversable so that reactants and products switch positions.
Reversable reactions do not instaneously come to a state of equilibrium. It does take a finite amount of time before that condition arises. When the rate of two opposing processes occuring simultaneously in the same system becomes equal then the system is in a state of dynamic equilibrium. There are two kinds of equilibrium, dynamic and static. Most physical and chemical reversable processes form dynamic equilibrium where the two opposing processes occur at equal rates.