The API is a collection of tools, definitions and protocols designed to assist in building and integrating new application software.
One way to think of an API is as a kind of menu; developers can browse through a list of available options for inclusion in their apps, along with descriptions of what function each option performs. They can then select the operations they wish to include based on functionality, without necessarily having to fully understand how these options are built or presented.
In other words, APIs allow developers to make more efficient use of their time, reducing the necessary amount of coding work and promoting consistency across all applications interacting with the same systems and platforms. API integration allows two or more APIs to easily share data and functionality with each other seamlessly and securely, and without direct human involvement.