For example, the number 9 appearing on a screen could mean an age, number of eggs, and so on. Humans can easily decipher the meaning from the surrounding natural language context, but natural language will always be difficult for computers to understand until they attain near-human intelligence. The digital signature technology is a data version of a hand-written signature. It addresses the problem of authorization and authentication of an electronic document. The private key analogy to hand signatures is that only you have the style, musculature, nervous system, personality, etc. to produce your signature, and for digital signatures, only you have access to your private key.
Digital certificates match a person to their signature.
By way of analogy, suppose you have a signed picture ID which identifies you as Harry Sig. The problem arises when someone questions whether this is your real name. A certifying agency is needed to certify that you are who you say you are. Depending on the level of security required, the certifying authority may ask you to show your birth certificate, signed social security card, your fingerprints, your tatoo, whatever, to prove you are in fact Harry Sig. The certificate authority would then sign a document with your picture and signature that you can use as certification of your identity and signature.