Encryption/decryption mathematically secures the privacy of communications. However, there is still potential for someone to modify the original message or substitute it with a different one; maybe in order to compromise your bank accounts. This is known as a Man in the Middle
(MITM) attack. Also, encrypting a large file or document using an asymmetric algorithm like RSA, can take a relatively long time.
One way of solving both these problems is to create a concise summary of the message to be sent, a digital fingerprint. This is known as message digest, one-way function or hash function. (This is one of the foundational protocols used in some blockchains; you will know it as - Proof of Work. For example, Bitcoin uses the SHA-256 Hash Algorithm to generate verifiably "random" numbers in a way that requires a predictable amount of CPU effort. This maths problem serves as a vehicle to confirm and record the transactions on the blockchain. See [insert article link] for a deep dive into ‘what makes Bitcoin... Bitcoin’).
Upon receipt of the message, the receiver creates their own copy and compares it with the one sent. If the summaries (hashes) are the same then the message has been received intact.
Asymmetric encryption/decryption gives us message security. Message Digests ensures message integrity. Now you need to ask, ‘How does your bank confirm the identity of you and hence the validity of your message?’ A process of authentication is therefore require.