What means "Data: Skeleton" in an .ogv file?
What is Ogg Skeleton?

From Ogg Skeleton 4:


  Ogg Skeleton provides structuring information for multitrack Ogg
  files. It is compatible with Ogg Theora and provides extra clues for
  synchronization and content negotiation such as language selection.
  The latest version of Skeleton, version 4.0, also provides keyframe
  indexes to enable optimal seeking over high-latency connections, such
  as the internet.


So basically the Ogg container can support multiple tracks of multiple track types types. However:


  Ogg does not know anything about the content it carries and leaves it
  to the media mapping of each codec to declare and describe itself.
  There is no meta information available at the Ogg level about the
  content tracks encapsulated within an Ogg physical bitstream.


Ogg Skeleton attempts to deal with these issues by providing information about the other streams that allows parsing and (de)muxing without having to decode.

What is stored inside such a stream?


  Ogg Skeleton is a logical bitstream within an Ogg stream that contains
  information about the other encapsulated logical bitstreams. For each
  logical bitstream it provides information such as its media type, and
  explains the way the granulepos field in Ogg pages is mapped to time.


See The "skeleton" meta information track for Ogg for more details.

Is it necessary to play the media file?

Continuing from the second quoted paragraph:


  This is particularly a problem if you don't have all the decoder
  libraries available and just want to parse an Ogg file (such as with file)  to find out what type of data it encapsulates.



For playback you are decoding.
You're using (libogg/libvorbis/libtheora and) the FFmpeg cli tools (or should be; I don't recommend Libav) to demux and decode. You have the various decoder libraries available. You can download a recent build of ffmpeg.
Your file is not complicated. It just contains one video stream, one video stream, and one data stream (the Ogg Skeleton). It does not contain multiple streams of video, audio, subtitles, etc.
Ogg Skeleton 4.0 can index keyframes for seeking in high latency situations. Given the information you (did not) provide I'm going to assume this does not apply to you.


So the answer here is: Ogg Skeleton is not necessary to play this media file.

You can test this by remuxing the file while excluding the data stream with a negative mapping:

ffmpeg -i input.ogv -map 0 -map -0:d -c copy output.ogv


Now try playing it.