Near the end of the Tales, at "Boughton under the blee," two mysterious strangers begin riding toward the group of pilgrims. When they arrive, the pilgrims can see that their horses are all lathered up, and their clothes are a-shambles: these men have been riding hard, almost like they're running away from someone. As we learn more about these two, we begin to have an idea of what they might be running from.
It turns out that the Canon and his Yeoman, or servant, are alchemists – people who try to turn metals like lead and iron into gold through chemical processes – and not particularly good ones. The Host slyly brings this to light when he asks why, if the Canon is really able to pave the road to Canterbury with silver and gold, as the Yeoman claims, he's dressed in such poor clothing. It turns out that the Canon talks a good game, but is never able to achieve what he attempts. In fact, says the Canon's Yeoman, they and their peers have often cheated people out of their gold with failed promises to increase it tenfold. This gives us some idea of why they might be in such a hurry!
The dynamic between the Canon and his Yeoman becomes unfriendly when the Canon hears the Yeoman spilling his secrets (and the truth of his ineptitude) to the Canterbury pilgrims. Despite his earlier stated goal of joining such a merry company, the Canon rides away in a huff, leaving the Yeoman to continue spilling the beans. It's not clear why the Yeoman is so eager to divulge trade secrets. Although some aspects of his narrative make us suspect that he's eager to get out of what is a thankless, frustrating trade, he ends by declaring that this science (alchemy) "wol us maken beggeres atte laste" (130).