"Cognitive process" refers, in basic terms, to the development of intellectual capability. "Sensorimotor stages" attempt to organize and order the physical capabilities and reactions to physical events.
Piaget felt both were essential components of infant development. As an infant's physical strength and motor coordination develops, the child becomes capable of different types of interactions with the environment.
As the child observes the results of those interactions, new understandings develop that allow the child to begin to think, plan, and react. Growth in either area is required to sustain development in the other.