The galactic plane is the plane where the disk of the Milky Way Galaxy lies.
Any object located perpendicularily above this plane is descibed as being X° (degrees) North, and likewise, any object located perpendicularily below this plane is described as being X° South.
While approaching the unusual dead star in 2269, the Enterprise experienced severe hypergravitional effects from the dead star's negative star mass, pulling it "two minutes right ascension" off course. As they entered orbit of the star Uhura detected a mysterious radio signal coming from an insectoid pod ship located at "nine seconds right ascension from galactic plane, dead ahead" of the Enterprise position in orbit of the star. (TAS: "Beyond the Farthest Star")