The social engineering toolkit is a program that already comes with Kali Linux. It allows one to study phishing attacks such as credentials harvesting, mass email phishing, and many more.
To use the program, open the program from the kali menu under Social Engineering Tools.
What’s an API?
API or Application Programming Interface is a computing interface that defines interactions between multiple software intermediaries. It allows for various programs, each running from a different computer while delivering the output to a single computer.