Infodump is one of those things that for some reason, writers tend to think it's okay to do. And it's not. There is never a time when it is okay to infodump.
Infodump is exactly what it sounds like. It's when an author tells you a huge amount of information (that you might or might not need to know in order to understand the book) in one big block of text or awkward chunk of dialogue. It gets the information down, yes, but it's dreadfully boring and often makes the info hard to remember. The author might interrupt a piece of dialogue between two sisters to explain the long and complicated history between them. The author might stop right before two enemies are about to duel to explain the significance of dueling in the kingdom.
And then, there's another type of infodumping. Dialogue infodumping. It might go something like this:
"How are you?" Mary Sue asked.
"Fine, fine," Gary Stu said. "But, as you already know, last week I broke up with my girlfriend, I'm haunted by dreams of Nerf snipers, and I'm still being stalked by a weremoose. So not all that well, actually."
|My face whenever a wild infodump appears.|
Did you catch it? That was infodumping. No, it wasn't a huge block of text. It's all in the "as you already know". Mary Sue already knows all of this information about Gary Stu. The reader doesn't, though, so one might think that dialogue is a good place to sneak that in. It's not a good idea, though, because it sounds stupid. If Mary knows all of this, Gary wouldn't tell her again. He'd have no reason to repeat himself.
Look, I made a list of people who enjoy reading "as you already know" type infodump:
So, infodump=bad. Fine. How do you avoid it, then? Sometimes a reader needs to know some information in order to understand the book. There's nothing wrong with that. At least, there's nothing wrong with it as long as you don't infodump.
You need to decide what information is crucial to your story. Do we need to know that the main character's best friend committed suicide a month before the story opens? Probably. Do we need to know that best friend's favorite color? Probably not. Take a good, long look at your story, and the information you're considering. If the story makes sense without including that information, don't include it.
The trick, then, to including that information is to sneak it in. You can't disguise infodump, but you can include that same information in smaller tidbits. It's like putting grinding up a baby aspirin and putting it into applesauce. A baby wouldn't take a whole pill, but if you smash the medicine into tiny pieces and put it in something tasty, they'll never know it's there.
Same thing with writing. We need to know some backstory, or something about your setting/characters/plot? Stick a line here, a hint there. A non-obvious reference in dialogue, as long as you aren't getting back into "as you already know" territory. A little bit of information here and there isn't infodumping, and it's a million times more pleasant to read. And you still cover the same info.
When sneaking that info into your story, though, be careful to continue to show everything, instead of telling. More on this here.
In summary, nobody likes long, boring infodumps. There are better ways to show that information (keyword: show) without intruding on the narration. Because it's not about you, as the author. It's about the characters.
PS: Shipping wars are still happening.