the easiest way to say it is that it is how and in what lang. the two devices will talk. Since their are a lot of different type and ways to talk, each device will try and talk in a certain order and when the two match that is how they will talk.
From what i can find out, it mainly deals with VOIP and phones,
Before you communicate with an entity you need to agree with your langage and characteristics. And even if you deal an agreement, you can change all this information at any time for whatever reasons you or the other entity want.
If i can add just thing if you are interest by VoIP, to better understand what signaling really is and why you need it. The best is certainly to come back to the source, and throw an eye on PSTN signaling methods and concepts which give you the core knowledge to better understand VoIP, it helps a lot.
Excellent analogy there Stephane. Much like here, it would appear that english is the appropriate signalled protocol. There are occasional posts written in other languages, but you significantly change the dynamics of who can or cannot respond based on that.
So any medium needs a signaling protocol to determine how the information will flow on it such that every endpoint can properly communicate. No agreement, no communications.
When you or anyone else makes a telephone call on a mobile phone it will go over the CNBAP = Common Node B Application Part, this is where the signalling protocols start being used for your call, they are used to setup the type of call you want and at this point they are common to all users hanging off this cell.
Your call will then go over the DNBAP = Dedicated Node B Application Part, this word dedicated is just what it means I.E. it is dedicated to you. Your call could be a voice or data call but still at this point the call is still setting up over the signalling protocols.
Once your call is setup you start talking, downloading your data or both for a multi call depending on the call you made. During the whole time you are in a call the signalling protocols are still going back and forth until you finish your call.
Should the signalling for some reason stop due to interference, cable breaks, card fails, thunder storms etc etc your call will drop because without the signalling protocols it cannot stay up or handover your call to another station as the signalling protocols control everything.