I´m the dummy.
I`ve really tried to understand this concept. I may be slow-witted. Would someone assume the huge task of explaining this concept for me?
 
Thanks
// Jan

All it means is you can set a SEND (to another bus) to be either pre or post your fader level.
 
In practical terms, and I'll use the simplest example I can think of, is when you're using a send to a reverb bus.
 
If your send is PRE fader, the reverb will not fade out when you lower the track fader
If your send is POST fader, it will fade out proportionally with the fader

Good Answer.

Bristol nails the most common example.
 
Another good one (esp. for us that have done live shows) is getting separate headphone mixes or stage monitors.  If you do a send prefader you can set the stage monitor (or headphone) mixes and not have to worry about moving the faders for the main speakers (or recording level.)  If you were to set up the monitor mix post fader, everytime you made a change to them it would change the repspective volume.
 
You can think of pre/post fader as where the signal gets tapped - either before the fader at a constant level or after the fader changes.
 
@

A slightly different example/use:
 
If you're using a send from (e.g.) a bass drum channel as a side-chain insert on a gate, to produce a pumping effect on another sound (a synth perhaps), then if the send is pre-fader, if you fade the bass drum out you'll still get the full pumping effect.  If the send was post-fader, then as you fade the bass drum out, the pumping effect would also decrease.

Thanks guys.
the problem was to understand the contexts in which one could use the concept, and your explanation made ​​sense . thanks

Also , this link is bookmarked on my browser:
 
Signal Flow Chart:
http://www.cakewalk.com/Documentation/default.aspx?Doc=SONAR%20X2&Lang=EN&Req=Mixing.07.html