ArcGIS Server allows you to apply detailed metadata to your services. To do this, you use the same metadata editing interface that you are accustomed to using in ArcMap. In many cases, the metadata that you have applied to your map or other resource is applied automatically to the service once you publish the resource. You can then edit the metadata as desired.
Why create metadata?
Metadata is information about your data. Metadata can help users know what's in your services, who created them, and what usage restrictions they might carry. Some organizations require specific items of metadata on any geographic information they publish. Even if you're not required to provide it, including at least some metadata can help ensure that your services are found, used, and attributed correctly.
Create metadata for services
In ArcMap, you can create metadata for any item by right-clicking the item in the Catalog tree and clicking Item Description. You can even do this for services listed under an ArcGIS Server connection. The metadata editing experience for services is the same as for all other items, and it allows for use of different metadata styles such as FGDC and INSPIRE standards. See the Editing metadata section in the ArcMap Help for full instructions.
If you attempt to edit metadata for a service and you see some metadata already, it's likely that the metadata was carried over from the originating GIS resource (such as a map document) when the resource was published as a service. Some resources are able to transfer their metadata to the service when published. However, after the service is published, there is no link maintained between the resource metadata and the service metadata.
For example, suppose you make a map document (.mxd) and provide description and contact metadata to it. You publish the map document as a map service. The map service has the same metadata as the map document at this point. However, if you edit the map service metadata, the map document metadata is not affected.
Access service metadata
Service metadata is available in ArcMap by viewing the service's item description as described above. The metadata is also available to web developers using a REST request. Developers can append /info/metadata to the end of a service's REST URL to get an XML file containing all the service's metadata. Developers can parse this metadata and present it to the end user of the service in the desired format.
When metadata is requested from a REST endpoint that is based on a service capability (for example, WMS, Feature Access, or Network Analysis), the metadata of the parent service is returned. For example, if you request metadata from a network analysis service that is enabled on map service Seattle, all the metadata for the map service Seattle is returned. If you have any important information to convey about the network analysis service, you should put this in the Seattle map service metadata.