The concept of sustainability covers a lot of ground. Put very simply, it means doing things in a way that, if you keep doing them that way, will continue to work.
An example of a Non-sustainable way of doing things is relying on petroleum to supply our energy needs. It is not sustainable because there is only a limited supply of it, created eons ago, which will run out sometime within the next few decades. An example of a sustainable way of doing things is using wind power, which is fueled by the Sun's direct energy, which can keep supplying us with power for millions of years.
Another example of Non-sustainable is rule by an authoritarian government. History has proven that this will always be overthrown by the people eventually, since people want and need to have a say in their own destiny. On the other hand, a truly democratic republic has the potential to keep going, since it allows the diversity of inputs, completeness of information, and consensus of at least a majority to come up with lasting solutions, stick with them as long as appropriate, and be flexible enough to adjust them if needed.
The more authoritarian a government, the more able it is to create/enforce laws in ways that benefit itself (its most powerful members) specifically, at the expense of the rest of the nation. This is analogous to an individual -- the more focused/obsessed an individual becomes with one particular aspect of life (e.g., work, kids, bank account, sex, drugs/drink, chocolate, etc.), the more all the other aspects of his/her life will suffer -- resulting in a lowering of the viability of the whole person. At some point the whole person can no longer sustain the one part that s/he was "living for."
However, the more evenly spread a person's energy is among all the things that make life (a) feasible (nutrition, water, shelter) and (b) worth living (chocolate, sex, creativity, spiritual communion with other living things, or whatever it is for that person), the stronger and more sustainable that individual will be.