What the result means
The Vertex is the western branch of the ecliptic's intersection with the local prime vertical. The Anti-Vertex is its exactly opposite zodiac point. The prime vertical is the great circle running through zenith, nadir, due east, and due west. The two displayed points are on the ecliptic; the Vertex is generally above or below the horizon and should not be mistaken for the physical west point on the horizon.
Because the local sky turns and the prime vertical depends on latitude, the birth time, historical clock offset, and both birthplace coordinates matter. The result uses the tropical zodiac and true ecliptic/equinox of date. The nearby-time table lets you check how much this particular geometry changes over an hour, but cannot recover a missing birth time. Near certain tropical locations and sidereal times, the two planes can nearly coincide and the Vertex degree becomes especially unstable.