What function does counter mode perform in a cryptographic system?

Counter mode (CTR) makes a block cipher into a stream cipher by generating a keystream block using a non-repeating sequence to fill in the blocks. This allows data to be streamed instead of waiting for blocks to be ready to send. It does not perform the reverse, turning a stream cipher into a block cipher, nor does it reverse the encryption process (decryption). Public keys cannot unlock private keys; they are both parts of an asymmetric encryption process.