Signaling refers to the exchange of information between call components required to provide and maintain service. As users of the PSTN, we exchange signaling with network elements all the time. SS7 is a means by which elements of the telephone network exchange information.
Information is conveyed in the form of messages.
Look for it on trunk 067. The called subscriber for the call on trunk 11 is busy. Release the call and play a busy tone. The route to XXX is congested. Please don't send any messages to XXX unless they are of priority 2 or higher.