When two thermally conducting substance with different temperature, come in contact with each other, the heat flows from high temperature to lower temperature. The flow of heat continues till the temperature of both the substance become same. This state is called as thermal equilibrium and process of obtaining thermal equilibrium is called zeroth law of thermodynamics.
Example: When person got fever, the temperature of the body is measured by thermometer. Thermometer is kept in contact with person, so heat from person’s body transfer into the mercury in the thermometer and the volume of mercury increases. When the temperature of the body and that of the mercury in thermometer becomes equal, the increase in volume of mercury stops and scale on thermometer is measure which shows the value to fever on person’s body.