What is said before the substance, to set what follows in place. A preamble is orientation, not content.
Medieval Latin praeambulum (a walking before), from prae- (before) + ambulare (to walk); what is said before the substance, walking ahead of it to set the ground it will stand on.
What is said before the substance, walking ahead of it to set the ground the rest will stand on. It orients; it is not itself content.