I've read fantasy books where I was totally immersed into the author's created world. I accepted it as real, because the author made me believe it. It was as real as my own world. I've also read fantasy books where I couldn't suspend my disbelief. The author was trying to get me into another world, but instead, I was watching the story from the outside, from the world I live in.
The difference between believable and not-so-believable fantasy is hard to pin down. There is a fine line between the two, and sometimes readers will disagree on what is believable and what isn't.
How, then, can you create a fantasy world that is just as real as Earth? How can you convince readers to put aside their disbelief?
(Oh, and in case you're wondering, "suspension of disbelief" refers to the reader putting aside their thoughts of "Yeah, that isn't possible" in order to enjoy a story.)
The trick to fantasy is that you have to make it more believable than the real world. That's the paradox of it. When you write a, say, contemporary romance novel, you don't have to convince readers that your novel's world is real. They'll simply accept that. In fantasy, though, you have to work for it.
You do this by "iceberging". It's a weird verb because I made it up. I don't know how else to phrase it. Ever heard of Hemingway's "iceberg theory"? It's like that. You write what is on the surface, but underneath there is a virtual mountain of things that are left unsaid but are still part of the story and the meaning.
From the way I see it, fantasy is the same way. You have to know your fantasy world inside and out if you ever want it to seem real. You have to know everything--big picture things, as well as smaller details. You amass countless bits of information about it in your head.
And then you don't use most of it. Think about it. If you use every single setting detail in your story, readers are going to be so bogged down with endless description that you won't have room for the actual story. No one wants to read every little thing about your setting.
You have to find a balance. Big-picture elements of your setting (i.e. kingdom A has a hundred-year-long feud with kingdom B) definitely need to come through. But you also need details. Details are what truly make your setting come to life.
If you intersperse details throughout your story without adding too many, it's like the tip of the iceberg. Readers won't get to know every single detail about your setting, but they'll get the feeling that this world is real and fleshed out. If done right, they won't feel like they've been dropped onto an alien plant with no help whatsoever.
In summary: Know everything you can about your setting, but don't use it all. Scatter details throughout, because details bring things to life and will create the impression that your world is real. But don't overdo it!