Upwelling is when cold deep water rises toward the surface. This process brings nutrients that are found deeper in the ocean to the surface which is often nutrient poor.
Deep ocean water is more nutrient-rich than simply because things (nutrients plankton carcasses fish carcasses) in the ocean sink.
Upwelling brings those lost/sunk nutrients back to the surface which creates blooms of algae and zooplankton which feed on those nutrients. These blooms then become feeding grounds for plankton feeders then fish etc sustaining that lives near the surface.