Your hard drive is the slowest part in your computer.
Programs tend to access the same files over and over again. Hard Drive Cache is a fast storage area where copies of files that the main program constantly needs to access can be stored temporarily. That way, when the program calls them, they're in something faster than the hard drive, ready to be used. Thus removing the slowest part of your computer from the loop.
The bigger the cache, the less often you computer has to access the harddrive, the faster your data access time.