A food chain represents the flow energy and nutrients through ecosystems, from start to end. It tells us how each organism in the ecosystem obtains the energy required for it to survive. At times, organisms may have a co-dependent relationship with other organisms, other times they would consume them.
It is the relationships that each organism has that allows the ecosystem to thrive. Should any part of the food chain be disrupted, the ecosystem could well collapse. As such, it is important for us to understand the intricacies of a food chain and to ensure that it is never disrupted irreversibly.