Threading facts into your novel

It's been a while since I've read Harry Potter, but I'm guessing she revealed details on things like Quidditch and House Elves when they were needed for the story. Revealing details about your world is great, but it's best to do it when it's relevent. It's not like you opened book one of the Harry Potter series and spent the first fifty pages reading up on the history of her world, the details were revealed in snippets when they became necessary for the reader to know. Try to tell the reader what they need to know, in that moment, for your story to be understood and to progress.

Like you said, focus on your plot and reveal what you need to when you need to.

I would say it is two parts timing and three parts subtlety. Obviously, information when it becomes relevant is your motto. Don't give us a monologue about the war until someone mentions the war. The rest of it, and perhaps where Rowling succeeds, is just in the little details. We learn that things can be enchanted by seeing things that are enchanted. We learn that Harry Potter is famous first through the reactions of other people to him. We see Harry eating chocolate frogs, Bertie Bott's Every Flavor Beans, and we get information from this. We see that we can enchant food to jump and ribbit. We have the technology to extract flavor from anything. We just learn a little about the Wizarding World's culture.

How you apply this to your book just depends on your story. Want to let people know mermaids exist without ever going to the beach? Have them use mermaid scales in a potion, or sew something with mermaid hair, or - if your world's a little darker - eat mermaid at dinner. Have them go to see a game of Quidditch or whatever at "____ War Memorial Stadium". Foreshadowing. We didn't know about Portkeys in Harry Potter until early on in the fourth, when they use it to get to the World Cup. The climax of the book depends on the use of a portkey. It's all about the little things she slips in early on and how they reflect on the story later, teach us things on a near subconscious level so that when we need to use a portkey to get Harry Potter to Voldemort, we don't say "oh, we can do that?" We know they exist, and accept the chance that the Trizard Cup is one.

Don't explain stuff merely because you think it needs explaining. Only explain it if it's gorgeous, thrilling, intriguing, or the reader absolutely is dying to know about it. Don't drop explanations on the reader until they actively want to know them.

Don't confuse world building with info dumping.

Info dumping is generally when the writer/narrator stops the action to explain something or give a history lesson - where not needed.

World building is woven into the story as the story progresses, building on settings and scenes as the characters arrive at them. There are many ways to show world building without stopping the story to explain...

Apply as needed. Don't lecture. Don't stop the flow of the story just to give background.

Best when it's coming from your MC and it's relevant to the situation at hand.