Co-signer refers to the concept of multi-signature cryptocurrency wallets. The general idea of cryptocurrency wallets allows single-key access, where the owner can access their funds using their own private key. But, looking from a security standpoint, these wallets might be insufficient to provide protection against cyberattacks once the information of the owner gets compromised.
Multi-signature wallets on the other hand require more than one individual to provide their own private keys to access the funds. The most common configuration of co-signer is 2-3, in which case two private keys are required out of three to access the funds. Co-signers are a fairly common concept in organizations, where crypto wallets hold large funds. Using a multi-signature wallet can protect its content from getting misused or outright stolen by a single employee.