Sri Lankan food is terrific, and it is often underrated by travelers who come expecting a pale imitation of Indian cuisine. It has its own distinct character: spicier than most, rich with coconut milk and tamarind, and built around a handful of essential dishes.
Rice and curry is the national meal and the default lunch across the island. You typically get a mound of rice surrounded by several small dishes — a lentil dhal, a dry vegetable curry, a fish or chicken curry, a sambal, and usually some pickled vegetables. The best rice and curry is found in small local restaurants (called "hotels" in Sri Lankan parlance) where the food comes out of big pots and costs almost nothing. Do not be put off by plain-looking places.
Hoppers (appa) are bowl-shaped pancakes made from fermented rice batter and coconut milk, crispy at the edges and soft in the middle. A plain hopper is good; an egg hopper — with an egg cracked into the bowl as it cooks — is better. String hoppers are a different thing: tangled nests of thin steamed rice noodles, usually eaten with coconut milk and curry for breakfast. Both are widely available and cheap.
Kottu roti is Sri Lanka's street food staple. A flatbread (roti) is chopped up on a hot griddle with vegetables, egg, and a choice of meat, and the noise of the iron blades clattering is the sound of every Sri Lankan town after dark. It is filling, cheap, and usually excellent.
Fish and seafood are outstanding on the coast — grilled seer fish (a kind of kingfish), crab, prawns, and red snapper prepared in various curries. Cuttlefish curry from the south coast is something worth hunting for.
Pol sambol — grated coconut with red onion, dried chili, lime, and Maldive fish flakes — is served with virtually everything and is addictive. So is the red onion sambol. Once you've had good sambol, food without it seems under-seasoned.
Food in Sri Lanka is hot by default. If you want it milder, say so before it is cooked rather than after. Most cooks will accommodate the request without offense, though in local places the concept of "a little less spicy" can be interpreted generously.