The text uses the analogy of mailing a letter to explain the concept of APIs, or application programming interfaces, which allow computer programs to communicate and automate specific tasks. It describes an API as an interface similar to a mailbox, where the sender forms a request (an addressed and stamped envelope) and sends it to a service (the postal system), which processes and delivers it to the recipient. The explanation highlights the value proposition of APIs by comparing the cost and logistics of mailing a laptop sticker to Lviv versus personally delivering it, emphasizing the efficiency and cost-effectiveness APIs offer by automating processes that would otherwise require significant resources. The text also touches on runtime considerations such as authentication, rate limits, and service level agreements (SLAs), which are crucial for maintaining reliable and secure API operations. It concludes by noting the universality of APIs, their various applications for different users, and the importance of having well-defined interfaces to facilitate seamless communication between applications.