In short a Cache is like the short term memory of an internet browser.
If looking for help instead of information: How do I clear my Cache?
Every internet browser is actually a program that uses the information you search to be more efficient and faster for you.
entireHR leverages this power everywhere in the system, one of the most obvious places you will see it is the first time you open the "Quick Booking" pop up, on the Allocations Screen, shown below. The very first time you open it may take 1 second to open, the second time about 0.5 seconds, the third about 0.2, the fourth time it's almost instant (eventually the speed will be maxed, but you get the point).
But why does it make things look weird or stop my software working?
Good question, basically when we do a any upgrade, the structure of the system changes, which means the browser suddenly cannot read the information as well as it could before. It's like a librarian that knows the location of every single book in her Library, especially the ones she loans out regularly (regular options you use in entireHR) when one day someone goes and swaps every book with the one on the left or right of it.
The librarian wouldn't be able to find anything! Not to mention probably have a bit of a panic attack in the process of frantically trying to put things back in order.
The same is true of the computer program, which is why you need to basically hit the "reset button" - which is done by clearing the Cache of your browser.
If you're wondering how to pronounce it, there is no set agreement with all "cash", "caysh", "kaysh" or even "cash-ay" being acceptable in the technical world.
If you want to learn even more about Caches Cache - Wikipedia