Ecological footprint is the impact each human, or group of humans, has on the habitat/environment they come into contact with. How much we consume, the waste we generate, the space we inhabit, all are considerations in an ecological footprint.
People in the world in general, and in America especially, consume way more than is sustainable. So it becomes a simple question of how many living organisms the ecology of Earth can sustain, also known as "carrying capacity".
There are several serious ecological risks we are running without much in the way of international efforts to mitigate them. The extinction rate of animals and plants is more than 1000X the normal background rate of extinction, some food chains and habitats are in continual disruption, with impacts we cannot even fully predict. The oceans are saturated with carbon, and biodiversity is diminishing. Some scientists suggest we are in the middle of the latest "great extinction", where 30 - 40% of the species on Earth may cease to exist within a century. The difference with this one, they say, is that it is both man-made and avoidable.