Of course, I made up some pseudo-intelligent bullsh*t answer and made a mental note to find out the REAL answer for next time someone asks.
It is best to answer truthfully and say that you are new to this and just know that it is better. The answer is that yeast is working on the beer and is cleaning up unwanted byproducts of fermentation which make the beer smoother and with less off flavors. There are a lot of posts about conditioning, aging and lagering beer on the web and on homebrewtalk. I suggest you read them and become informed. The best part is that after a while you will see for yourself the effects of aging beer and that is the best teacher.
Fermentor 1: Bill's House Ale II, Fermentor 2: German Helles, Fermentor 3: Bill's Schworzbier (Black Bier)
Tap 1: Bill's House Ale II, Tap 2: German Hefewizen, Tap 3: Nut Brown Ale
Future Brews: Stone IPA Clone, Blonde Ale, Budvar Clone, Newcastle Clone
New toy: Blichmann 27 gallon fermentor
If you find yourself in a hole, the first thing to do is stop digging
Good judgment comes from experience, and a lotta that comes from bad judgment