The assignments section tells Procmail where to find things, such as your mailboxes, or programs that it might need to run. Here are descriptions of the assignments in the excerpt above. This tells Procmail where to look for other programs. Directories are separated by colons. This specifies the name of a file to use as a usage log, to which Procmail will write any diagnostic or error messages during its activity.
Recipes are where the real work of filtering is done.
The flags and lock-file business I'll cover later. The idea is that if the conditions are met, the action is performed. The action could also be an address to forward the mail to, or a program to start, or even a block of commands. We'll see more complex examples later. The condition tells Procmail what to look for in a mail message.