The calculations involved in RSA encryption and decryption operate on numbers. For this reason the plaintext message M has to be transformed into numbers before it can be encrypted. Using a RSA modulus N only the numbers 0,1,2, ..., N-1 can be encrypted, which necessitates splitting up the message into blocks of length k. Block length k always defaults to the maximum possible length of ASCII character strings with which messages can be coded into numbers from the range 0,...,N-1 without loss of any information.
You can also specify shorter block lengths by clicking on Options for the alphabet and the number system. In this dialog, options are also provided to choose how the numbers should be presented and to experiment with the "Dialogue of the Sisters" story from c‚t magazine.
Instead of entering text, you can of course also encrypt a number sequence m = m[1] # m[2] # ... # m[j] directly.