Simply put, 'explain' is used to explain a given schema type. You can define your own metadata using the md-[namespace|object|resource]-create commands, or just use it to look at the default schemas. Example arguments for 'model' would be something like 'metadefs/namespace', 'metadefs/resource_type', or - most simply - 'image'.
An example:
[stack@osp7 ~]$ glance --os-image-api-version=2 explain image
+------------------+-------------------------------------------------------------------------------------------------------------------------------------------------+
| Attribute | Description |
+------------------+-------------------------------------------------------------------------------------------------------------------------------------------------+
| container_format | Format of the container |
| min_ram | Amount of ram (in MB) required to boot image. |
| ramdisk_id | ID of image stored in Glance that should be used as the ramdisk when booting an AMI-style image. |
| locations | A set of URLs to access the image file kept in external store |
| file | (READ-ONLY) |
| owner | Owner of the image |
| id | An identifier for the image |
| size | Size of image file in bytes (READ-ONLY) |
| os_distro | Common name of operating system distribution as specified in http://docs.openstack.org/trunk/openstack-compute/admin/content/adding-images.html |
| self | (READ-ONLY) |
| disk_format | Format of the disk |
| os_version | Operating system version as specified by the distributor |
| direct_url | URL to access the image file kept in external store (READ-ONLY) |
| schema | (READ-ONLY) |
| status | Status of the image (READ-ONLY) |
| tags | List of strings related to the image |
| kernel_id | ID of image stored in Glance that should be used as the kernel when booting an AMI-style image. |
| visibility | Scope of image accessibility |
| updated_at | Date and time of the last image modification (READ-ONLY) |
| min_disk | Amount of disk space (in GB) required to boot image. |
| virtual_size | Virtual size of image in bytes (READ-ONLY) |
| instance_uuid | ID of instance used to create this image. |
| name | Descriptive name for the image |
| checksum | md5 hash of image contents. (READ-ONLY) |
| created_at | Date and time of image registration (READ-ONLY) |
| protected | If true, image will not be deletable. |
| architecture | Operating system architecture as specified in http://docs.openstack.org/trunk/openstack-compute/admin/content/adding-images.html |
+------------------+-------------------------------------------------------------------------------------------------------------------------------------------------+