The message to be sent is called ``plain text''; the encrypted version is called ``cipher text''. Let's assume that Alice is to send a message to Bob.
First Alice and Bob need to agree on a secret key of 256 bits. This might be sent via a Public-Key system, for example, or produced via Diffie-Hellman or a variant.
Then Alice represents her message in bits (perhaps using the ASCII code) and breaks it into 128-bit blocks. If the message length is not an exact multiple of 128, the last block can be padded. In the use described here, Alice runs each block separately through the algorithm to encrypt it and then sends the encrypted block as ciphertext.
Bob decrypts the blocks using the same secret key and a procedure that amounts to running Rijndael backwards.