MD5 is one of a number of cryptographic fingerprinting algorithms. The notion is that you can take an input string of arbitrary length, and generate a 128-bit "fingerprint" identifying the string. According to RFC 1321 (where MD5 is defined), "it is conjectured that it is computationally infeasible to produce two messages having the same message digest, or to produce any message having a given pre-specified target message digest."