Included free with the base game — no Steam store pageFree for years now, this DLC pulled the British dominions out of their supporting role and gave them a voice of their own — before this, Canada, Australia, New Zealand, India and South Africa were little more than territory flying the UK's flag.
What New Content It Added
The pack split its additions between five dominion focus trees and an autonomy mechanic that matters in any UK game, whether you play them directly or not.
Nations: the Five British Dominions
Canada, 
Australia, New Zealand, British Raj 
India and South Africa each get their own focus trees, with a relationship to the metropole that's more or less tense depending on the case.
Mechanics: the Autonomy System
The core mechanic is the autonomy system: dominions start with different levels of independence from the UK and can gain more room to maneuver over time, up to full independence in some cases. It also adds technology sharing between members of the same faction, something that didn't exist in a structured way before.
If You Play the UK
If you're running the UK, this DLC barely registers day to day but weighs on every game: your dominions contribute manpower and industry based on how much autonomy they have, so managing them well affects your whole campaign even though you don't control them directly. It's free, so there's no purchase decision — but the autonomy system is still, today, the foundation of how any colonial empire functions in the game.