An ecological footprint is a measure of human demand on nature. It is a tool that assesses the productive surface area required for a population to meet its needs in terms of resource consumption and waste absorption.
Imagine that you are Robinson Crusoe, isolated on a desert island: how big would your island have to be (land, lagoon and accessible sea included) for you to be able to live self-sufficiently over the long-term, and meet all of your needs in terms of food, heating, building materials, fresh air, drinkable water, waste absorption?
This surface area represents the ecological footprint of our Robinson Crusoe. Obviously, if the lifestyle of our castaway exerts too much pressure on his island (for example, if he makes big camp fires every night to stave off loneliness), i.e. if his ecological footprint is larger than the size of his island, he eventually risks jeopardising his survival over the long term...
Globally, humanity's ecological footprint is an estimate of the amount of biologically productive land or marine area needed to meet all our needs.