This tool lets you understand the process of AES encryption.
For Germans it can also be interesting to watch my presentation with which I explained several encryption methods in school.
To follow the (by the way fantastic) explanations of this site,
you could use something like this where the first parameter is your plaintext and the second is the passphrase:
python visualizer.py "ATTACK AT DAWN!" "SOME 128 BIT KEY"
If you only want to test the encryption of this tool or you already know how AES works you might want to use this instead:
python encrypt.py "ATTACK AT DAWN!" "SOME 128 BIT KEY"