In the context of the full article (
https://en.wikipedia.org/wiki/Iridium_satellite_constellatio...), it's clear they're talking about the polar orbits used by the Iridium constellation, which have "seams" around the Atlantic and the Pacific as the "first" set of satellites passing north-to-south overlap with the "last" set of satellites coming back south-to-north on the other side of their orbits. So of the 6 orbital planes used by the Iridium satellites, each plane covers 1/12th of the globe for each "half" of its over-the-poles orbit. So there are two "seams" where handoff is not supported, one off the eastern seaboard and one roughly over Japan.
There's an animation on linked article that explains this pretty well: https://upload.wikimedia.org/wikipedia/commons/thumb/9/90/Ir...
