It means that the ownership of the contract of the token has been renounced. The contract is basically all the information and rules for a token. If the ownership has been renounced no one has access to the contract to adjust it. Meaning no changes can be made to the contract in order to scam, like to make it untradable or changing the taxes. This does not mean it is always safe, as certain scam tactics may have been implemented before renouncing it, or by creating backdoors.
You will only come across this in De-Fi memecoins trading as renouncing ownership also means no improvement or additions can be made. In general it is a way to make buyers trust that the token is relative safe and decentralized.