The phrase "heavy lifting" refers to a task or job that requires significant effort, energy and concentration. It is often used in reference to physical labor, such as moving heavy objects or performing strenuous exercise. However, it can also be applied metaphorically to describe any challenging mental or intellectual work, such as analyzing complex data or solving difficult problems. In either case, the phrase implies that the task at hand is demanding and requires a significant investment of time, energy, and resources in order to complete successfully.