Mappings are a way of constructing more complicated data types by allowing nested key-value pairs. For example, from the same system.site block from earlier:
page:
type: mapping
label: 'Pages'
mapping:
403:
type: path
label: 'Default 403 (access denied) page'
404:
type: path
label: 'Default 404 (not found) page'
front:
type: path
label: 'Default front page'
When the type is set to mapping, the schema expects one or more children defined under the mapping attribute. These could potentially be mappings as well, allowing for arbitrary nesting depth.