The majority of strings can not be compressed. By string I just mean a binary number, which is ultimately what all computerised stuff is.
How come? Well, say you wanted to compress a 4-bit string to a 3-bit string. There are 24 possible 4-bit strings, but only 23 possible 3-bit strings: there are twice as many possible 4-bit strings as 3-bit ones.
Compression is a lossless process so that the compression can be reversed, so each uncompressed (4-bit) string should match up with a unique compressed (3-bit) string, in a one-to-one correspondance. Yet there are only half as many 3-bit possible strings ...