A SIM card is the removable chip that identifies a subscriber on a mobile network. It stores the credentials a carrier uses to authenticate a line, such as subscriber identifiers and cryptographic keys, so the network knows which account is allowed to connect. In practical terms, it is what ties a phone number and service plan to a specific user or device.
In cyber security, SIM cards matter because control of the SIM can become control of the phone number. Attackers may try SIM swapping or port-out fraud by convincing a carrier to move a number to a SIM they control. That can let them intercept SMS-based login codes, reset passwords, and access accounts protected by text-message MFA. Defenders reduce this risk with carrier account PINs, port-out locks, stronger authentication methods, and by avoiding SMS as the only second factor.