Wallets that are stored offline and can only be connected to the internet through physical effort (eg. plugging in the USB, turning on a hardware wallet). This provides an extra layer of security compared to wallets that are stored on devices connected to the internet (eg. phone wallet).
An acronym for Zero-Knowledge Succinct Non-Interactive Argument of Knowledge, zk-SNARKs refers to a protocol where one can prove posession of a given piece of information (eg a string or hash) without revealing that information and also without any interaction between both the prover & verifier.