From a technical perspective, according to eIDAS, “‘qualified electronic signature’ means an advanced electronic signature that is created by a qualified electronic signature creation device, and which is based on a qualified certificate for electronic signatures”.
From a legal perspective, QES has the equivalent legal effect of a handwritten signature, i.e. a “special legal effect”. QES offers a high level of assurance that the named signatory is in fact the person who signed the document. What this means in practice:
If the validity of a Qualified Electronic Signature is legally challenged, the court may presume that signature is valid and that the signatory has been identified. Thus, the initial burden of proof rests on the challenger (who repudiates their signature) to show that the document was not signed by the named signatory. By contrast, in the case of a document/agreement signed using a (simple) Electronic Signature (ES) or an Advanced Electronic Signature (AES), the burden of proof may be reversed: the party defending the validity of the signature needs to demonstrate that the named signatory did sign it.
In effect, the special legal effect that QES enjoys offers some benefits to the party relying on the signature; mainly that the threshold for repudiation by the signatory can be perceived as higher than if a ES/AES was used. Nevertheless, a court must still weigh the evidence at hand and in e.g. a case of identity fraud even a QES can be deemed invalid – just as a contested ES/AES may still be deemed valid even though the initial presumption of validity didn’t apply. The quality of evidence related to the signature process is of course important in this context, not the least when ES/AES is used.