XMP metadata is descriptive information about the file embedded in the PDF, for example the name of the author, keywords, copyright information... XMP refers to labeling technology that is used, i.e. Adobe's Extensible Metadata Platform. You can view the XMP metadata in a file's Document Properties in Acrobat (Click the Additional Metadata button, then select Advanced).
PDF/A supports the predefined schema definitions of the XMP 2004 specification. However, as these only cover general metadata requirements, custom XMP schemas ("extension schemas") are also supported to meet company- or industry-specific requirements, on condition that a description of these extension schemas is embedded in the XMP.