Efficiency is nothing but the ratio of output from the system to the input provided to the system. Thermal efficiency is calculated for work-producing devices like engines, turbines.
Usually, the output is the thermodynamic work produced by the system and the input is heat input into the system. The higher the efficiency the better. Many engineers and companies try to improve the efficiency of the machines because if the efficiency is higher it can be said that, for the same work output the more efficient machine needs less heat input. The heat added into the system comes from natural resources and it cost us money, as well as these resources like coal, oil, natural gas, are depleting day by day.