The main hook to "Salmon Run" is that in addition to fending off waves of enemies, called "Salmonids," you and your teammates will also need to collect a number of "Golden Eggs," which drop from bigger foes, within a set time limit. Even after you collect all the eggs, however, you still have to survive the remainder of the wave. The mode also has a difficulty setting that goes from 5% (easiest) to 100% (hardest). Even on the easiest difficulty, you'll need to work with your teammates to call out bosses and incoming threats.
During the presentation, Nintendo showed off a bit of gameplay for Splatoon 2's new four player co-op mode, Salmon Run. It looks like it'll function quite similarly to standard horde modes, but with the added wrinkle of Splatoon's unique mechanics. You can check out the video below: