By definition, affinity is:
How likely your audience is to like a given page compared to everyone on Facebook.
That means if you see a very high affinity score, it will be extremely likely that the audience you selected is also liking those pages. Though affinity score is not much important for targeting and getting a better result in your ads, marketers use it simultaneously.
In April 15, 2017 marketers didnt get the affinity score in audience insights. Most of them thought it was a bug but finally, Facebook declares officially that they will not provide affinity score anymore.
That made a huge buzz in facebook marketing world especially among the noobies (as most of the pro marketers dont depend on affinity) So many random posts about affinity made me feel that people depend on affinity score for their targeting which is not good and smart.
But magically last night it appears in audience insights and just like before people making buzz again. Hell yah, its back, here is your affinity. She is back after a long vacation.
But dont depend on it. A smart marketer never depends on any tools no matter what. Even audience insight disappear tomorrow you have to keep calm and carry on. Good luck.