<p>Related to my <a href="https://stackoverflow.com/questions/28975/anyone-using-couchdb">CouchDB</a> question.</p>

<p>Can anyone explain MapReduce in terms a numbnuts could understand?</p>

Take a bunch of data
Perform some kind of transformation that converts every datum to another kind of datum
Combine those new data into yet simpler data
Step 2 is Map. Step 3 is Reduce.
For example,
Get time between two impulses on a pair of pressure meters on the road
Map those times into speeds based upon the distance of the meters
Reduce those speeds to an average speed
The reason MapReduce is split between Map and Reduce is because different parts can easily be done in parallel. (Especially if Reduce has certain mathematical properties.)
For a complex but good description of MapReduce, see:
Google's MapReduce Programming Model -- Revisited (PDF)
.