Something that is "sustainable" can be continued for a long time.
The important thing when you're preparing for a race is to pick a sustainable pace. Don't run so fast that you tire yourself out and have to stop.
"Sustainability" is a noun that describes this idea of being able to continue something for a long time. But more specifically, we often use this word to talk about environmental sustainability. That means the ability of the Earth's environment to continue to function well.
As an example, imagine a company that manufactures paper. At that company, there might be a "Sustainability" department which makes sure that the company uses more recycled paper, and that more trees are planted to replace the ones that they use to make the paper.
You can use the word "sustainability" like this:
We need to think not only about short-term profits, but also about sustainability.