A cryptographically relevant quantum computer is a notional quantum computer that would be able to break classical encryption keys in a reasonable amount of time.

A working CRQC would be able to break classical public-key cruptography (e.g. RSA), allowing attackers to decrypt communication that was previously secure.

Symmetric key crapytography (such as AES), with sufficiently large key-sizes, is entirely safe from quantum computers. [^1]

[^1] : https://words.filippo.io/crqc-timeline/