Wiktionary
underscoring
n. An underline. vb. (present participle of underscore English)
Wikipedia
Underscoring
In film production, underscoring is the playing of music quietly under dialogue or a visual scene. It is usually done to establish a mood or theme. In a play, sometimes incidental music is used for this purpose.
Usage examples of "underscoring".
But the strongest lines, those underscoring his determination to maintain American neutrality, were delivered directly for the benefit of Congress and the foreign diplomats present, not to say the opposition press.
Seating herself at the little white and gold spinet, the Princess vArian began to pick out a tune, the sugared notes softly underscoring the well-modulated babble of Exalted voices.