SOAP UI is a tool used to generate mock web services from a WSDL file for testing client applications before the actual web service is available. It allows you to obtain a WSDL, create a SOAP UI project, generate mock services, test queries, and configure mock service responses. You can start by getting the WSDL for a sample weather web service, generate a mock service, start the mock service to test requests, and configure responses by populating values or adding logic.