Digitizing a pattern (be it text, a picture, music, whatever) means transforming it into a single number. Quite often a very, very large binary number, but a number nevertheless. You can think of it as the page number of the book that contains all possible patterns.
Data compression is the black art of finding a smaller page number in a smaller book that corresponds to the original number. If you then give someone the smaller page number and the name[1] of the smaller book, they can find the original page in the Big Book, and therefore retrieve the original pattern.
[1] The name of the book is a symbol for how the book is defined, i.e. exactly what subset of the BookOfBooks are contained therein.
See other InterestingMemes.