Phytoplankton is the foundation of the ocean food web. But it needs two things to grow: sunlight and nutrients. Sunlight is plentiful on the ocean surface where plankton float. Nutrients, not so much. Instead, they are at the bottom of the ocean where decaying organic matter sinks down to.
This is where upwelling kicks in. When strong winds blow across the surface of the ocean, it pushes water away. This water is then replaced by deeper water from beneath, bringing nutrients with it. These nutrients act as fertilizer for phytoplankton, increasing their growth, and subsequently the fish and other marine feeding activity across the food web.
One of the greatest places to experience upwelling is the California coast near San Francisco. Here, upwelling produces nutrient rich waters that provide a smorgasbord of good marine eating.