Scarcity is what forces you to make trade-offs. Suppose you have an economy that produces and consumes 2 products, A and B. In a world without scarcity, you have enough resources (land, machinery, raw materials, manpower) to produce as many of each product as you need/want. However, in a world with scarcity, you have a limited amount of production resources. You can produce, let's say, 10 A products or 10 B products, or a combination of both products with less than 10 products each. For every additional A product you produce (up to the max of 10), you have to produce less B products. This is a trade-off.
