What is "high german consonant shift"

In historical linguistics, the High German consonant shift or second Germanic consonant shift is a phonological development ( sound change) that took place in the southern parts of the West Germanic dialect continuum in several phases. It probably began between the third and fifth centuries and was almost complete before the earliest written records in High German were produced in the ninth century. The resulting language, Old High German, can be neatly contrasted with the other continental West Germanic languages, which for the most part did not experience the shift, and with Old English, which remained completely unaffected.