“The plain of Tabor” is an older rendering of a phrase that should probably be translated “the oak of Tabor.” It designates a landmark rather than a plain.
The oak of Tabor is mentioned once, in the account of Saul’s homeward journey after Samuel anointed him. Saul was directed to pass this landmark on his way.
The expression has been corrected in the Revised Version because the underlying reference is understood to be an oak, not a plain. It was probably the same general landmark associated with Allon-bacuth.