Web of Time

The Web of Time is, apparently, the orderly co-existence of events within Time.

During his sixth incarnation, the Doctor explained that, for example, to destroy the planet Earth in 1985 would disrupt the Web of Time. (DW: Attack of the Cybermen)

Earth existed, in history, after 1985, so to destroy Earth would change the course of history. The Doctor, then in his first incarnation, may have meant this when he stated to Barbara Wright that she could not change history. (DW: The Aztecs)

The Doctor would speak of the Web of Time thus:

"The Web of Time could not exist until the great Rassilon built the Eye of Harmony, the hitching post of chronology, that which does not whither nor change its state." (BFA: Neverland)

The Doctor implied that one of the primary functions of the Time Lords was the maintenance of this Web.He regarded this duty with grave seriousness.