A digital signature is a mathematical scheme used to verify the authenticity and integrity of digital messages or documents. It functions like a handwritten signature but is much more secure. Digital signatures use cryptographic techniques to create a unique code that is linked to the document and the signer, ensuring that any changes to the document after signing can be detected. When a sender signs a document digitally, they use a private key to create the signature, while recipients can use the sender's public key to verify it. This process is essential for secure communications in various applications, including email, software distribution, and financial transactions.