Humans have needs of goods and services. They use them and there is a volue that other scientist in human sciences study. Economists will study their exchange value. Since the needs are unlimited, there can be a lot of exchanges!
But the reality is different. we live in a world of scarcity. We cannot satisfy all our needs. We lack of everything: time and money and all that we can buy with it.
Scarcity is different for different people, different countries, different centuries. My ancestors who arrived in Québec city in 1643 did not have the quality of life that I have now. People living in developing countries don't have the lifestyle of western countries people, even after the USA financial crisis. And many people in USA are suffering now from lack of everything in the rischest country of the world for the whole human history.
Scarcity is universal.
But scarcity is not absolute. There are a lot of wealth in the world and that wealth is growing every year all the time. So, scarcity is not absolute, but relative to something. Relative to what? To the needs!
Economic scarcity is this relationship between the illimited human needs and the limited ressources available to satisfy all the needs of everybody all the time.