Does it mean that the voltage and frequency are stable, and not moving too much up and down? And if it happens, can it damage something? I hear all the times things like "Check if the overclock is stable", and i want to know what it means...
And besides that, does unstable frequency have any effects?
And what about voltage? Of course that unstable voltage is not good, but what is considered as an unstable voltage?
Constantly changing frequency and voltage is just fine. I leave SpeedStep enabled to let it do just that, for lower heat output. No harm in it at all.
It may cause instability at extreme overclocks, but at any normal OC, it's just fine. And it definitely won't damage anything. It's meant to do that.
The only thing that may be a problem is if the VRM's aren't great, so they won't (or can't) provide stable load voltage, but there's nothing you can do about that, except get a higher quality motherboard.
And just two final questions
1) Is there any way to check if i need less or more voltage?
2) I know it may sound stupid, but if the cpu has spare voltage, will the cpu consume more power than what it is supposed to consume without spare voltage, or it will consume more power?
About question number 2, i mean that if the cpu gets for example 1.3v and it realy needs only 1.25v, will it pull more power?
Yes it will. It'll use the voltage that it gets, no matter what it needs and use more wattage accordingly. Now, you can start off at a low voltage (just high enough to POST) and work your way up to fully stable voltage to help that.