Okay, so imagine you have a bunch of different sounds that you use when you talk. Some of these sounds are made by using your voice, like "mmm" or "zzz" or "th" (like in "think"). Other sounds are made by moving your lips and tongue around, like "p" or "t" or "f".
Now, a long time ago, people in Germany used these sounds differently than we do today. They would say things like "p" instead of "f", and "t" instead of "z". But then something happened that changed the way they spoke - it's called the High German Consonant Shift.
Basically, over time, people in what is now Germany started using their mouth and throat differently when they spoke. They started making some sounds in a different way, and that made other sounds change too. So instead of saying "p" like they used to, they started saying "pf" (like in "pfennig"). Instead of "t", they started saying "ts" (like in "zehn"), and instead of "z", they started saying "s".
This might seem like a small change, but it actually had a big impact on the German language. It made German sound different than other languages that didn't go through this shift, and it made it easier for people to understand each other when they spoke.
So that's the High German Consonant Shift - it's basically a change in the way people in Germany made sounds when they spoke, which had a big impact on the German language.