A transit of Earth across the Sun as seen from Neptune takes place when the planet Earth passes directly between the Sun and Neptune, obscuring a small part of the Sun's disc for an observer on Neptune. During a transit, Earth can be seen from Neptune as a small black disc moving across the face of the Sun.
Naturally, no one has ever seen a transit of Earth from Neptune, nor is this likely to happen in the foreseeable future. Nevertheless, the last such event took place on August 11, 2006 and the next will be a grazing transit on January 23, 2081. The next central transit takes place on January 25, 2082.
A transit could be observed from the surface of one of Neptune's moons rather than from Neptune itself. The times and circumstances of the transits would naturally be slightly different.
Transits of Earth from Neptune are empirically observed to occur in clusters, with two such clusters every 80 years or so. These clusters occur whenever Neptune is closest to the ecliptic because the ecliptic is the plane of Earth's orbit and planetary transits only occur when planets are aligned.