Frop is the herb that "Bob" smokes in his pipe and the reason he is grinning. Technically its name is "Habafropzipulops". It only grows under very special circumstances, like in Yeti droppings, for instance. Many SubGenii enjoy smoking frop as a way to get their Slack back from the Conspiracy.
Here is a video of Rev. Sinphaltimus Exmortus using Time Control to smoke frop in his pipe with time going backwards (look at the direction the smoke goes in) - this shows some of the magical powers of frop: