The term "flat" is referring to the response. The response is how a speaker recreates the signal.
For instance your stereo system in your house sounds different than the system in your car. Mostly because the speakers are "calibrated" or "tuned" for that type of response.
Studio monitors are "tuned" so that they sound more like what you are hearing, rather than some nicely fine tuned, high coralation, & high fidelity DCM, or Sony loud speakers with bass response adjustments and dolby sound options. The "after market" speakers have much more "color" than any true near field monitor.
And if you were to mixdown on some Cerwin Vega's, it might sound AWESOME on those speakers, but sound like ass on everything else.
The deal is, if it sounds good on the near field monitors (with the flatest response) then it should sound fine on all the other brands of speakers that are what we refer to as having "dynamic response".

Exactly. Monitors = speakers. Flat = what their frequency response graph should look like.

I'm sure you've seen a frequency response graph before - pretty much all microphone and speaker manufacturers include them with their products. It compares volume/sensitivity whatever at different frequencies, or pitches. Are high sounds louder than low sounds, etc?

Loud
------------------------------------
Quiet
Low Frequencies High Frequencies
Ideally, thats what you wanna see. :) But a lot of consumer speakers hype the "high lows" - ie. 80-120 hz is boosted significantly in relation to other frequencies - and the "low highs" - 6khz - 10khz - which on a cheap system is often quite distorted. You want your mixes to sound good on these systems.. but also on other systems with different responses. So, get speakers that have as balanced a response as you can afford. If possible, see a frequency response graph for whatever speakers you are looking into purchasing. Figures like "40hz-20khz response" are useless unless they specify the variance within that response. A good, relatively "flat" monitor would be +- 2 db within that range. Well, that should get you started.

Thanks guys. I am begining to understand what you mean.

G'day

Go to my post under the thread above 'the importance of great monitors"

It's on page 6 I believe, and would save me re-typing the whole thing :)

Bare in mind also a 'flat' speaker system is not everything. There are many other factors involved as the above post will detail.

Also, when you put a pair of 'flat' frequency response monitors in a 'not flat' response room, you get new issues to deal with in the form of uneven bass tones which can play havock with even the bestest systems out there.

Thus the importance of room treatment, to create as 'flat' as possible a room response.