Waveform

According to the First Doctor, points in history became fixed once the waveform collapsed. It was dangerous at this point to interfere. (AUDIO: Daybreak) Such fixed points could be brought about, for example, when learning about the nature of one's own death. The Eleventh Doctor explained that time could not be rewritten once its details had been read. "Once we know what's coming, it's written in stone." (TV: The Angels Take Manhattan)

The Vardans could take the form of waveforms, and hence pass through physical barriers. (AUDIO: Wave of Destruction)

Behind the scenes
The First Doctor, here, is alluding to, as studied in quantum mechanics, which occurs when a wave function in a (ie. with multiple possible quantum states) is observed, and thus collapses into only one possibility.