In short
A food chain shows who eats whom, as a line of arrows from a producer to the animals that eat it. A food web joins many food chains together.
Producers, such as green plants, make food by photosynthesis. Consumers eat other organisms: primary consumers eat plants, and secondary consumers eat primary consumers. Decomposers, such as bacteria and fungi, break down the dead. Energy is lost as heat at each step, so only about passes on to the next level, and food chains are rarely longer than four or five links.
Grass grasshopper frog snake hawk. If of energy is in the grass, only about reaches the snake.