What is a digital signature?

A value produced by signing a message with your private key that only you could create, yet anyone can verify against your public key.

It is unforgeable (nobody can derive your key from it) and publicly verifiable (tampering with the message makes verification fail). This is how the network knows a transaction spending your funds really came from you.

Go deeper:

• Digital signature — Wikipedia — diagrams Alice signing and Bob verifying with her public key.