Every economic agent experiences scarcity. There are three types of economic agents: households, companies and the public sector.
Economic agents have a limited amount of resources, specially money and time, that they can dedicate to purchase goods and services in order to satisfy their needs.
Therefore every choice, involves rejecting another alternative that is valuable in terms of individual preferences (attractiveness for the individual), money and time. This is called opportunity cost and it is beared any time that a choice is made due to the scarcity of resources, and it is defined as the value of the best alternative that has been rejected when the choice is made.