One of the base principles of cryptography is that you can't just encrypt multiple messages with the same key. At the very least, what will happen is that two messages that have identical plaintext will also have identical ciphertext, which is a dangerous leak. (This is similar to why you can't encrypt blocks with ECB .)
If you think about it, a pure encryption function is just like any other pure computer function: deterministic. Given the same set of inputs (key and message) it will always return the same output (the encrypted message). And we don't want an attacker to be able to tell that two encrypted messages came from the same plaintext.
The solution is the use of IVs (Initialization Vectors) or nonces (numbers used once). These are byte strings that are different for each encrypted message. They are the source of non-determinism that is needed to make duplicates indistinguishable. They are usually not secret, and distributed prepended to the ciphertext since they are necessary for decryption.
The distinction between IVs and nonces is controversial and not binary. Different encryption schemes require different properties to be secure: some just need them to never repeat, in which case we commonly refer to them as nonces; some also need them to be random, or even unpredictable, in which case we commonly call them IVs.