I got a Boss Me-20 pedal for christmas from friends and i love it but i still have questions regarding the clean sound( i'm actually pretty unexperienced regarding gear xD ) For instance, is the clean sound only an amplified version of the strings vibrations ( without any modulation or distortion) or does it modify it? Also is it possible to have the clean sound of my amp (MG 30dfx) and still have the me-20 on?

Please help me understand more... thx

oh yeah, merry christmas...
The clean sound you get when you have your ME-20 on (without any effect on) is an amplified version of the sound produced by the string vibrations that went through the pedal's EQ. That's basically it.

Regarding your amp question, it is possible to have the clean sound of your amp with the ME-20 on. Just turn on the tuner (press the OD and Mod buttons at the same time) and the pedal just bypasses the signal directly to your amp (which means that all the tone you are getting comes from your amp only).

Fender American Standard Stratocaster
Fender Hot Rod Deluxe
Boss ME-20
Boss DS-1
Dunlop Crybaby Wah
Washburn South Side Jr.
Dunlop Jazz III XL picks

R.I.P. Rick Wright
i've never used that particular pedal, but with other multifx units it was possible to maintain your amps original clean sound, or make your own clean sound with the pedal.
MY Music
Chute CC-04 2x12
Fender American Deluxe Strat SSS (with DG-20's)
Martin Dx-1
Big Muff Pi Tone Wicker
Keeley Mod Bluesdriver
Holy Grail Plus Reverb
MXR Carbon Copy Delay
Boss RC-2 Loop pedal