Pluto represents deep transformation: the crises that make you mature, power (undergoing it or wielding it), what must fall away so that something else can be born. It moves so slowly (up to more than 20 years in some signs) that it marks almost exclusively a generation — it is the house it occupies that makes its reading individual.
Discovered in 1930 (and reclassified as a "dwarf planet" in 2006, a status that changes nothing about its use in astrology), Pluto does not belong to the set of seven classical planets and therefore has no essential dignity in the Hellenistic system, for the same historical reason as Uranus and Neptune.
Modern astrology conventionally assigns it co-rulership of Scorpio, a sign previously governed by Mars alone — a widely shared convention, postdating classical dignities.