The keys of public key cryptography are very large numbers, sometimes a thousand or more digits long. A fingerprint is a much smaller number or set of numbers and letters that can be used as a unique name for that key, without having to list all of the key's digits. So, for instance, if you and a friend wished to make sure you both had the same key, you could either spend a long time reading off all the hundreds of digits in the key, or you could each compare your key's fingerprints instead. The fingerprints presented by cryptographic software usually consist of around 40 letters and numbers. If you carefully check that a fingerprint has the right value, you should be safe against impersonation using a fake key. Some software tools may offer more convenient alternative ways to verify a friend's key, but some form of verification needs to happen to prevent communications providers from easily being able to listen in.