This page contains a list of all the Liquibase XSD files. A new version of the XSD is generally available for each change to the major or minor version of Liquibase. XSD schemas are written in XML, and describe what a properly formed XML document for a particular application should look like. If you really want to get the lowdown, this is a good article on XML and namespaces.
Simply put, referencing one of these XSD files in your changelog allows both Liquibase itself and any third-party XML tool (like an editor) to validate that your changelog is properly formed. There is more information about the format of XML changelog files in the databasechangelog page and in the FAQ.
These XSD files are embedded into the Liquibase jar file, and don’t normally need to be downloaded from this site, but these are here for easier reference.