Simply put, illustrations that are submitted by well known, famous artists and later got picked up and recognized by various major publishing companies around will be labelled as an originator.
Usually these illustrations are popular, and it generated lots of views and marks given by various users throughout.
The alternative usage to "originator" would be "Birthplace".
Usage at pixiv / How to identify an "originator"
Tags that are labelled as 本家, 元祖 (ganso) or 本人 (honnin, lit. "Original person") are the famous illustrations found not just at pixiv, but also other magazines, CD cover, or video sharing websites (Youtube, NicoNicoDouga). One of the best example would be Black★Rock Shooter, which pixiv was the origin birthplace of this craze.
Japanese defined "Originator" as:-
1. Tags that depicts well known illustrations that started a craze that everyone follows. One illustration that started it all. An illustration that spawned, started, or triggered various parodies or meme also works the same way. Many illustrations that are found in famous PV of Vocaloid songs fall into this category.
2. Established/famous artists or mangaka who submits their illustrations in pixiv are also labelled as Originator under this category. Sometimes they will also use tags like ”プロの犯行” (puro no hankou, lit. "Professional's offense") or "本人" to identify such illustrations.
This tag is usually used for the latter.