If something is sustainable, than it can be maintained or continued. A 5-minute mile pace probably isn't sustainable for anyone other than Olympic marathon runners.
These days this adjective is often used to describe something that has a low long-term effect on the environment, like "sustainable agriculture" — a method of farming that doesn’t ravage the land, deplete resources permanently or produce a great deal of pollution. Break down the word into two parts, sustain and able, to visualize and remember that you're able to sustain something sustainable.
