Reiko Asakawa: So that video is.
Ryuji Takayama: It's not of this world. It's Sadako's fury. And she's put a curse on us.
Yoichi Asakawa: You know what, Mother?
Reiko Asakawa: Yes?
Yoichi Asakawa: Tomo-chan watched the cursed video.
Yoichi Asakawa: Tomo-chan told me to.
Answer: The other answer is not correct, although you could take it that way if you wanted. The novel upon which "Ring 0" is based was not even out at the time, nor was the prequel even planned at the time this movie came out. So that's not really the answer, although you could retroactively try to connect the two. As for the actual question: the crawling figures are typically viewed as being representations of the victims of the volcanic eruption that Shizuko (Sadako's mother) predicted. Especially as they appear right after words like "eruption" appear onscreen. Or they can be viewed somewhat more nebulously as representations of Sadako's pain, or the pain her victim's feel.
TedStixon