It depends on what discipline you have been trained for, as clearly indicated in Wikipedia. As Mechanical Engineering Schools have several departments concerning solids, fluids, thermal, operational, instrumentation and many more, your job is to keep the machines working properly and efficiently or you may develop new products or you may have some other function as suggested by the mechanical engineering jobs. Sometimes mechanical engineering is overlapping with other engineering disciplines like manufacturing, aerospace, maintenance, chemical and many more but always you must have good knowledge of mathematics, physics, mechanics, computations, materials, professional design software etc. Training on the job is a priority for young mechanical engineers who need time and effort to get established and of course they need lots of good luck.