I'm not sure you're going to get a better explanation than what's in the book, which is excellent. These are sort of easy, sort of standardized ways way to create interfaces between different systems. For example, PayPal supports a SOAP interface in which case you can use the SOAP capabilities included with Web2py to interact with the PayPal system.
XMLRPC was gaining popularity awhile back but not so much anymore. More info: http://www.xmlrpc.com/ JSONRPC is similar but uses the JSON data format instead of XML. JSON is easier to work with, especially with JavaScript.
