Furious 7 was for Paul -- a dedication card at the end of the last Fast and Furious installment read, "For Paul," honoring the late Paul Walker, who died during production. The Fate of the Furious, meanwhile, pays its respects to Brian O'Connor, the FBI agent-turned-street racing outlaw Walker played in six movies.
Spoilers for one major plotline in F8 below.
The setup for the tribute is a bit involved, but the gist of it is this: Cipher (Charlize Theron) is a megalomaniac hacker who holds Dominic Toretto's (Vin Diesel) son hostage in order to force Dom to do her bidding. Yup, Dom has a son, and the mother is Elsa Pataky's character, Elena, who early on informs Dom that she calls their son Michael (his middle name) because a father should be the one to name his child.
If you're muttering, "Ho'dup...I'm going to need a little more than the gist of this. Dom has a kid now? Since when? And it's not with Letty? Also, who's Elena?" just know that you are in good company (My company. Because I was perplexed as f**k, too). The timeline of this is a bit unclear: Elena Neves is the Rio de Janeiro police officer Dom fell for in Fast Five and later split with when his longtime love, Letty (Michelle Rodriguez), returned from the presumed dead. (Elena became a DSS agent alongside Dwayne Johnson's Luke Hobbs.) As Elena explains in F8, she found out she was pregnant around the time Letty returned -- meaning she likely got pregnant sometime between F5 and F6 -- and was planning to tell Dom after his honeymoon (which begins F8). However, F6 is set only a few months after F5 and at some point in there Tokyo Drift takes place, chronologically, which means that Elena had at least two movies' time to inform the father of her baby she was pregnant and also means she was certainly with child in Furious 7 when Deckard Shaw (Jason Statham) detonated a bomb that blew her and The Rock out a window and multiple stories down onto a SUV. Right around this point is when my brain seized and I realized I'd spent more time doing the math on this than I guarantee any of the Fast and Furious writer did. (The timeline is, honestly...wild.)
As far as Brian himself is concerned, he drove off into the proverbial -- and literal -- sunset at the end of Furious 7, retiring from any and all duties deemed fast or furious. He is briefly mentioned in Fate when Letty refuses to contact him or Mia (Jordana Brewster) after Dom goes "rogue."
During Fate of the Furious' final scene -- one of those dusk-set family gatherings the movies love to wind down with -- Dom raises his son for the ever-growing Toretto team, Lion King-style, and says, "Meet Brian." It's a sweet tribute, only slightly diluted by the fact that Brian is still alive and technically still exists in the F&F universe -- i.e. Brian Michael is just named after Dom's retired brother-in-law. (Admittedly, the gravitas I'm searching for would mean that Brian would have had to die in F7 and it would have been real tough to watch Walker's character die in any similar way to the actor at the time -- and now, as Diesel recently told ET, "I'm still reeling from the loss, the idea of continuing along without my brother, Pablo, and what that felt like and the complexities of that.") Anyway, it's nice. A bit confusing, but nice.