Known as one of the OpSigs, this is one of the most critical op_codes in the scripting language. In short, it verifies that a provided signature is valid given a public key & returns a value of true (1) or false (0). It's worth noting that based on the transaction structure (legacy/segwit/taproot), the serialization of the message/transaction or the signature scheme itself (ECDSA vs. Schnorr) may differ. CheckSig is used in most of the common scrits including P2PK & P2PKH.
OP_Code Walkthrough
Verifies a cryptographic signature against a public key and a message.
Pop top item (Public Key)
Pop top item (Signature)
Check signature verification (ECDSA | Schnorr)
Push verification result (0 or 1)