In relation to human activity, it is no more than a slogan - a misleading string of words that ignores the reality that all human activity affects the environment in some way that contributes to an irreversible change. Some human activity has less short-term impact but the very existence of humans and our inability to live in a way that does not convert things into a non-naturally recyclable form means that 'Environmental sustainability' is a myth or, at best, an exaggeration.
We can reduce our impact. We cannot stop it so it can't be 'sustainable' in the long term unless we choose to focus on a small fragment of the big picture (such as 'plant a tree for each one we cut down). There's nothing wrong with that but, at the same time, who is to say it is wrong for humanity to carry on the way it always has? Who says it has to be sustainable? Mankind might die out along with some other species but the planet will survive and will establish a new equilibrium eventually. Wouldn't that be a GOOD thing, to give some other species a chance at dominance?