What's actually happening
Pluto's orbit is long and elliptical. A full circuit takes about 248 years, and the time it spends per sign ranges from roughly 12 years to nearly 30. Because retrograde motion cycles it back across sign boundaries, an ingress typically unfolds as a sequence of crossings over one to three years before the final commitment.
Aries is the first sign of the tropical zodiac — the sign of initiation and of Mars.
What the tradition makes of it
Pluto in Aries transforms the nature of beginning. The way power is exercised through individual assertion, the way new things come into the world, the relationship between individual will and collective force — these are the domains that undergo the deepest and most irreversible change during this multi-decade chapter.
The previous Pluto in Aries transit ran from approximately 1822 to 1851 — the era of revolutions across Europe, of the emergence of the modern nation-state as a vehicle for popular will, of Romanticism as the cultural expression of the transformed individual. The pattern does not repeat exactly, but the rhyme is recognizable: the radical restructuring of what it means to assert, to begin, to claim the right of individual force.
How to actually use it
This is generational weather, not daily forecast. The Pluto in Aries chapter is the background against which several decades of individual lives will play out. What is worth attending to is where the transformation of initiation and assertion surfaces in your own life — where the old form of beginning is no longer available and the new one has not yet been named.
When in doubt
What is changing about the nature of beginning itself — in your domain, in your lifetime?