A PIN code (Personal Identification Number) is a short numeric password for identity verification, playing a key role in SIM card and smartphone security.
SIM PIN is a 4-8 digit code set on the SIM card. When enabled, it's required every time the phone powers on. Even if your phone is stolen, the thief can't use calls or mobile data without the SIM PIN. Default values vary by carrier - change it to a number only you know.
Three consecutive wrong SIM PIN entries lock the SIM, requiring a PUK code (PIN Unlock Key) - an 8-digit number from your carrier. Ten wrong PUK entries permanently disable the SIM, requiring replacement.
The screen lock PIN is separate from the SIM PIN. Screen lock PIN unlocks the phone itself; SIM PIN authenticates the SIM card. Setting both provides double-layer security.