The greater the access the cryptanalyst has to the system, the more useful information they can get to utilize for breaking the cypher.
The receiving party uses an inverse cipher to decrypt the ciphertext to obtain the plaintext.
A secret knowledge is required to apply the inverse cipher to the ciphertext.
This secret knowledge is usually a short number or string called a key.
It is usually assumed that the encryption and decryption algorithms themselves are public knowledge and available to the cryptographer, as this is the case for modern ciphers which are published openly.