While a standard definition of sustainability is difficult to come across and agree upon, sustainability is largely the idea of people coexisting with the Earth now and in the future. Sometimes you see sustainability used in terms of having three distinct categories, which are environmental, economic, and social.
Environmental Sustainability centers around the idea of keeping Earth's ecosystems in balance. In a sustainable world, humans would use natural resources no faster than the Earth can reproduce them. Often environmental sustainability takes the form of limiting climate change, preventing the loss of biodiversity, reducing pollution, among other things.
Economic Sustainability would mean that everyone had the financial means to do all of the things that they needed to. This would ensure that everyone had access to food, water, shelter, and healthcare.
Social Sustainability is the idea that everyone has universal human rights and equal access to necessary resources. Social sustainability seeks to remove barriers between people and necessities. This can take place in many different ways, such as improved access to educational resources or more political empowerment to underrepresented groups.