Info-dumping is one of the worst sins you can make as a writer.
But what exactly is info-dumping? And why is it so bad?
It is the process of giving information clumsily and inappropriately. For sure you need to tell the back-story, describe your character's personality, and the story's setting. However, all this information should be elegantly woven into the story, not pasted arbitrarily on, or dumped. (Hence the phrase info-dumping because you're dumping information on the page.)
It's bad because it's jarring for the reader. It takes her out of the story to a certain extent because it's communication from the author rather than the characters. It's a form of telling-not-showing.
Here's the start of a story (my story Grace Under Pressure, in fact):
It was a sunny April day in Ireland, and I had just got dressed up. I hoped my husband Dev would appreciate this - he didn't seem to appreciate me much any more. It was only a cheap dress, seeing as we were broke now.
Now, there's nothing wrong with that as such, but it's flat, and the information is given very bluntly.
Here's a better version, which is the version that actually begins the book:
I gave myself a final, critical, scrutiny in the mirror, turning this way and that to view myself from different angles, and I gave myself a nod of approval. I’d do. I still scrubbed up reasonably well, I was glad to note. Dev would surely have to notice how much effort I had made. He would definitely pay me some sort of compliment.
I was wearing a brand new dress (bought at one of the less-expensive chains, given our current financial situation, but even so I was pleased with it). It was a dark pink halter-neck with a wide skirt and tons of netting underneath. I wore a cream crocheted shrug over it, given that it was only mid-April. The weather was exceptionally good mind, but still, mid-April in Ireland? How warm could it be?
The second example is still only 141 words long, but it gets in all the important information: the fact that she's feeling a bit neglected by Dev, the fact that they're financially challenged, the fact that it's April in Ireland. (It's always important to get the time and location established early.)
Do you see, though, how much more subtly it gets these facts across? They're slipped in almost incidentally. The detail about the shrug, for example, not only gives us a better mental image of the heroine's appearance, but allows us to comment on the fact that it's April and Ireland, rather than say so straight out.
The second example didn't manage to give us the information that the heroine was actually married to Dev, but that was okay - the information wasn't relevant right away.
Now, of course there will be times for giving straight-forward information, times when you just cannot do it any other way. But do try to slip the relevant information in subtly whenever you can - it makes for much more professional and glossy writing.
"As you know, Bob ..." Info Dumping
"As you know, Bob ..." is another kind of info dump. It's when the information is dumped in dialogue rather than narration. So:
The worry about Mary is the current situation, possibly even the dramatic question, and the fact that the speaker met her in college etc, is the back-story. But this is a hugely clumsy way to tell that back-story - indeed, it's classic info-dumping.
The whole point is that Bob clearly does know this, and so the speaker wouldn't even refer to it.
Never, ever, have characters tell each other stuff
they know already.
Of course they can refer to it, just as we real people do - but make sure it's done in a realistic way. Here's an example from my book Yellow Brick Road. The situation is that Laura has just asked her boss to agree to a project, and he's dubious:
“Yes,” she answered. She knew exactly where this was heading. Was she never to be allowed forget about it?
“Now it didn’t matter, honestly it didn’t. They were nearly all very nice about it. And as for that group which was quite nasty, well, you always get some awkward ones.”
Do you see the way the specifics of what Laura did were never mentioned, but yet we, as readers, were able to figure it out?
That's a much better way of dealing with information.
I recommend that you read some novels with a view to seeing how the writers got across the information the needed to get across. Was it done elegantly? Or clumsily? And what elements made it elegant or clumsy?