This is an 'encryption key' assigned to a specific person. The signature is made up of a very long binary number that is unique to them.
If that person signs an electronic document with their digital signature, then it is legally recognised as a valid signature (at least in the UK).
'Signing' in this case means to encrypt part of the document with their signature. Anyone decrypting that part must use the same key, so proving who signed it.
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Click on this link: Digital Signature