Final answer:
The subject of a message advises the recipient on the content and importance, enabling them to prioritize their response.
Step-by-step explanation:
A message's subject advises the recipient on how to treat the contents of the message. The subject line of an email plays a critical role much like an essay title; it should concisely represent the main idea or point of the email. Good subject lines are specific enough that the recipient can easily refer back to, and understand, the email's purpose at a later date. When crafting a subject line, the goal is to ensure it's clear and indicative of the email's contents so that it advises the recipient of the email's importance and helps them to prioritize their response.