If we damage the planet by taking too much too quickly it may never recover.
There will not be enough food left for the Earth's growing population.
The ingredients we need for our factories will run out. This way of acting is 'unsustainable' because it can not last.
If we take fewer resources or take them more slowly the Earth has a chance to recover.
There will be enough food and we will always have some ingredients for our factories to make things we need.
This way of acting is 'sustainable' because we can keep doing it.