What is the SOAP API?

Simple Object Access Protocol (SOAP) is a web communication protocol designed by Microsoft, SOAP API is used to expose web services and transmit data over the HTTP/HTTPS. 

SOAP only supports the XML data format and follows the present standards of messaging that includes encoding rules and convention to provide procedure requests and responses.

References: 

https://www.soapui.org/learn/api/soap-vs-rest-api/