Poke (pronounced "poh-kay") is a traditional Hawaiian dish of raw fish, almost always yellowfin tuna (also known as 'ahi tuna'), marinated in soy sauce, sesame oil and chili sauce. It's frequently served with fresh avocado as well, or in lettuce cups. Basically, it's like a ceviche, but with a great ginger-y, spicy incredible flavor.
Ahi tuna can be quite expensive - many recipes call for a pound of the stuff, which runs anywhere from $16-$20 for sushi-grade here. I usually make poke in smaller quantities, especially since I can usually purchase ahi in individual servings, vacuum-sealed and frozen, from stores like Whole Foods & Sprouts. I also try to go with the most sustainable option whenever possible.
The traditional recipe usually calls for green onions. I'm not a fan of them, but, if you want to keep things traditional, just add 2-3 Tbsp of chopped green onions to the marinade mixture.