XML targetNamespace

"When we want to [validate an XML doc], we need to identify which element and attribute declarations and type definitions in the schemas should be used to check which elements and attributes in the instance document. The target namespace plays an important role in the identification process."

-from w3c here

Qualified and qualified elements are discussed on this page under the section "Qualified or Unqualified."

Qualified: "all the elements and attributes in the instance must have a namespace, which ... adds namespace complexity"
Unqualified: "only the globally declared elements and attributes in the instance must have a namespace, which ... hides the namespace complexity from the instance"

The meaning of global is described on this page:

"When an element declaration is a child of the xs:schema element, the declared element is global. Global elements can be referenced by other element declarations, allowing for element reuse."

More on global vs. local here.