What does "zero offset between two waypoints" mean in ICAO Doc 4444?
GNSS units have the ability to fly an offset, meaning a track parallel to the one drawn between 2 waypoints. For example a left offset of 5 NM means the track is now 5 NM to the left of the original track and running parallel to it.

The lateral separation minima specified in 5.4.1.2.1.2 for two aircraft (both GNSS) on intersecting tracks require that neither be flying such an offset – since the tracks intersect at a common waypoint that is used for separation, an offset would change this common waypoint. Here's a crude representation (blue track is an offset):