Info-dump: Large chunk of indigestible expository matter intended to explain the background situation. Info-dumps can be covert, as in fake newspaper or "Encyclopedia Galactica" articles, or overt, in which all action stops as the author assumes center stage and lectures. Info-dumps are also known as "expository lumps." The use of brief, deft, inoffensive info-dumps is known as "kuttnering," after Henry Kuttner. When information is worked unobtrusively into the story's basic structure, this is known as "heinleining."
from Bruce Sterling, "The Turkey City Lexicon", Paragons iteration.
Perfect examples of info dumps done wrong can be found on CSI shows—where one character explains the scientific details of an experiment to another character who should already know that information.
Sometimes you have vital information that you need to weave into a story. The temptation is to just tell the reader the details and get if over with. This can lead to long passages that stall out your story. Too many of these, especially in the beginning, will stop the reader completely and they'll never get around to finishing the book.
The trick is to determine which details are vital and which are fluff, and to weave those details into your story in a way that moves the plot and action along.
Here is a good example of how to do that.
Another good discussion of how to avoid info dumps is found HERE. I recommend you read the entire article because she has some good ideas on avoiding the info dump. However, I'm reposting an excerpt as your writing prompt/excercise for today:
Too much background? Do the Q&A test.
A test to find out if you've put too much data in the story is to read it yourself and, paragraph by paragraph, underline any background information and write in the margin what question it's intended to answer.from Finessing the Infodump by Paula Fleming
After doing this, look through the questions. Are some listed more than once? If your protagonist keeps flashing back on degrading episodes he's suffered because he's illegitimate, perhaps most of those flashbacks can be taken out -- your reader is screaming, "I got it, already!"
Also, are all of the questions important to the story? The history of the Fifth Dynasty may be a fascinating tale in itself, but perhaps all the reader needs here is to see a portrait of Emperor Archibald IV hanging over the current emperor's throne, a daunting presence overlooking an insecure ruler. (Question: Why is the protagonist's father unwilling to admit he made a mistake?)