It's content that "expands" existing relationships by adding events to friendships, rivalries, and best friends. That's only the core idea though; in terms of gameplay the benefits you get from friends may be a few events that help reduce stress to a minimal extent and a few modifier bonuses here and there. The content that comes through these events is merely a tiny flavor text event that pops up to "attempt" to get you immersed. I'll admit, my imagination is good enough to get immersed in the junk I'm consuming but this is all outweighed by the bane of my existence in CK3: House Feuds. Any modifiers or immersion is just outweighed by the never ending spam of house rivalries that I've gotten my playthoughs. I love the of concept of house rivalries but this was just poorly implemented. You can't control what your house mates do of course when they end up murdering some other house or slightly them in a way the triggers the house feuds.
Example of "lovely house feuds breaking"
I didn't mind getting a house feud every 25 years but that's when one-sided house feuds would trigger where you can't end a house feud till you end every house member in that house in order to "end" a feud. It got to a point where you would get 10 - 15 houses targeting your house and well, eventually the house feud was with my own house somehow. As you would expect, I wrote this entire review out of frustration of the house feud mechanics after my house dynasty was screwed over house feuds everywhere including my house vs. itself.
So what's the main idea?
Pros:
- You get some event content to immerse yourself in
- Modifiers can be beneficial
- House feuds can add to immersion if unbroken and run periodically
Cons:
- House feuds can break the longer you play (spam house rivalries that you can't end or might even turn your house against itself too)
- Content is minimalist flavor text
- Modifiers are barely beneficial and barely impact your game-play
TL;DR:
Friends & Foes DLC is not worth it. Don't waste money on it expecting more immersion or gameplay mechanics. It's just flavor text.