The overt moral lesson in "The Pardoner's Tale" is that greed is the root of all evil, as it is explicitly stated by the pardoner. In addition, gluttony, drunkeness, gambling and swearing are each discussed in the "Prologue to the Pardoner's Tale" as moral vices to be avoided.
The Pardoner, who tells the tale, travels preaching against these vices through biblical tales. The tale itself is about a literal search for death personified. When three rioters go looking for death, their plan is subverted and then fulfilled by the desire to possess a pot of gold. Their greed ultimately causes each of the rioters to die, thus affirming the moral of the tale regarding greed.