For example, the word dog is a noun, right? But what about this sentence:
We just built a new dog house.
What is the word dog doing in the sentence? It's modifying house!
English does this a lot--and if you take any news article off of Yahoo! News you will find several examples in it. Here are a few from this article about bad traffic jams:
[The traffic jam] was simply the result of too many vehicles clogging the road, particularly a bevy of heavy trucks carrying construction supplies into Beijing, ironically for road work that was intended to help ease congestion.
Here the word road is modifying work, just as if it were an adjective.
Here is another from an article about Luke Skywalker's pants for sale at an auction:
The pants have been on the auction block before.
Here auction, the noun, is modifying block.
Lastly, (SPOILER ALERT!) here's an example from an article about the untimely demise of the beloved Chief Webber on Grey's Anatomy:
Grey’s Anatomy fans will have an emotional season’s finale, as another tragedy is lurking in the corner as season 9 of the hospital drama comes to a close.
The word hospital is totally a noun, but what is it doing? It's modifying drama!
So when do you do this?
Well, usually when there is not an adjective form of the word already. For example, the word difficulty is a noun, but there is already an adjective form, difficult. Therefore, you wouldn't say, *It was a difficulty problem (I mark ungrammatical sentences with a *).
However, if there is not already an adjective form of the noun, then you can use the noun to modify other nouns. Take the word diet, for example. You can be on a diet. It's a noun. There is not a separate adjective form, per se, so that's why we find things like Diet Coke, and diet pills, diet plan, the diet industry .
Anyway, read a few news articles and look for this. It happens a lot.
Have a good one, and good luck out there.
 Davies, Mark. (2008-) The Corpus of Contemporary American English: 450 million words, 1990-present. Available online at http://corpus.byu.edu/coca/.