iCM metadata is made up of properties and values.
Properties can be thought of as a category of values that can be used to describe something. Values make up a property and can be related to other content items in iCM.
For example "colour" could be a property, with the values "red", "blue" and "green". "Height" could be a property with the values "short" and "tall". "For sale" could be a property with the values "true" or "false".
Metadata properties, like other iCM content, can be organised into groups. Groups are used to organise properties within iCM and allow you to restrict user access to the properties within them using iCM Privileges.
iCM metadata should not be confused with the metadata titles, descriptions and keywords which are included in the source code of a web page. While some templates or site themes may use iCM metadata for this purpose, it has a far wider range of uses, which will be described in the documentation of the GOSS product you are using.