In order to be useful, metadata needs to be standardized. This includes agreeing on language, spelling, date format, etc. If everyone uses a different standard, it can be very difficult to compare data to other data.
A key component of metadata is the schema. Metadata schemes are the overall structure for the metadata. It describes how the metadata is set up, and usually addresses standards for common components of metadata like dates, names, and places. There are also discipline-specific schemas used to address specific elements needed by a discipline.