XML is eXtensible Markup Language, a document processing standard. 
XML Forms are forms based on XML files.

Sesame version 2.x uses XML to export form designs and application designs between different databases and applications.

So, iin Sesame, XML forms could be considered as forms that were created by doing an XML export of a form design, and then importing that XML file into Sesame creating a new form like the original one.

Here is a link to an on-line tutorial: http://www.w3schools.com/xml/xml_whatis.asp