The word "brood" is used to personify the billboard by attributing a human-like quality to an inanimate object. Personification is a literary device that gives human qualities to non-human things, in this case, the billboard is given the quality of brooding, which is typically associated with deep thought, contemplation or worry. The use of "brood" suggests that the billboard is thoughtfully considering a message or idea and may be reflecting on it in a contemplative or melancholic manner