A schema is a set of property definitions. For instance, the Nuxeo Platform uses the Dublin Core schema for most of its documents. A document that holds the Dublin Core schema has the following properties: a title, a description, a creation date, a modification date, an author, a subject, etc. The schema is a nice concept to enable property definition re-use on several document types.
You can create here your own schema, with custom metadata. Choose the type of the metadata as well as wether it is simple or multi-valued. You can also edit the validation rules( https://doc.nuxeo.com/display/NXDOC/Field+Constraints+and+Validation) for that metadata. Pay attention to the metadata type that can be among standard scalar type as well as Nuxeo Platform specific rich types: Directory, Users/Group or Document. Choose one of the latter ones if the value of your metadata is a reference to such an entity. Doing so, you will be able to benefit from automatic configuration of your widgets, as well as some advanced features in the REST API as described in https://doc.nuxeo.com/display/NXDOC/JSON+Marshalling#JSONMarshalling-document-JSON-extended-fieldsDocumentJSONandExtendedFields