Complex sentence: Their role is to phagocytose (engulf and then digest) cellular debris and pathogens, either as stationary or as mobile cells, and to stimulate lymphocytes and other immune cells to respond to the pathogen.
Split into simpler sentences: Their role is to phagocytose (engulf and then digest) cellular debris and pathogens, either as stationary or as mobile cells. They also stimulate lymphocytes and other immune cells to respond to the pathogen.