What is it?
Digital signature is like a regular signature, but instead of your hand and pen - you're using your ethereum account. Yeah, it may sound like a magic, so let's dive a bit deeper.
How they are used?
Digital signatures are used to prove account ownership. In fact, when you're trying to send some funds - your wallet does nothing except creating and signing the transaction. That's how the cryptocurrency works in general, but how can to sign a message, not a transaction?