Short Answer
A metadata standard defines what information about other data is recorded and how this has to be done.
Detailed Answer
For metadata to be machine-readable, it has to follow certain rules. Metadata defines categories, and the standard gives those a standardised format and standardised names.
For example, when the identification date of a species is recorded, in the Darwin Core metadata standard the corresponding field is </dwc:dateIdentified> and the allowed format is the ISO-8601 format, e.g. 2000-01-01. Therefore, a metadata standard specifies exactly how the metadata has to be recorded, including which information, which formal type (numbers, text, format) and how many characters may be entered in the respective fields.