A food web is a series of interwoven food chains. Food chains overlap within an ecosystem (a community of plants and animals and their physical surroundings) because many organisms eat a variety foods rather than a single species of animal or plant.
For example, squirrels and mice eat several species of plants, as well as insects. The predatory broad-winged hawk feeds on insects, mice, and small reptiles. And foxes hunt a variety of creatures from worms to rabbits.
Animals that eat a variety of plant and/or animal species have a greater chance of survival than those that have a single food source. Complex food webs provide greater stability to an ecosystem.
Sources: Barrett, James A. Biology, pp. 937-38; Cunningham, William P., et al. Environmental Encyclopedia, pp. 344-46; Ward, Jack A. Biology Today and Tomorrow, pp. 421-22.