Fundamentally, a XAML page describes the classes that the runtime should create, the property values and event handlers for the instances of the classes, and an object model hierarchy—that is, which instance is the parent of another instance.
All XAML documents are well-formed XML documents that use a defined set of element names. Therefore, all rules regarding the formation of well-formed XML documents apply equally to XAML documents. For example, the document must contain a single root element; all element names are case-sensitive; an element definition cannot overlap another element definition but must entirely contain it, and so on. If you're not familiar with XML syntax, now is an excellent time to learn it.