A food web is a system of interlocking and interdependent food chains. A food chain is a series of organisms each depending on the next as a source of food.
Basically, it's a large system of life in pictures.
The Diagram
The primary producers, trees, plants, etc, gain their energy from the sun. They utilize photosynthesis, which is the reaction on transferring the sun into energy for the plants' survival.
The primary consumers, usually herbivores, receive their energy from consuming the primary producers.
The secondary consumers, which could be omnivores or smaller carnivores, gain their energy from eating primary consumers.
Tertiary consumers, the biggest and worst of all the predators, consume all the levels of animals, even the producer level, if they are omnivorous.
The decomposers/detritivores gain their energy by consuming all dead organic matter. When the predators leave an animal carcass, or the primary consumers don't fully consume a producer since it was attacked by a predator, the detritivores consume them both, cleaning the forest and also keeping life intact.