An email subject is a concise and attention-grabbing line of text that summarizes the main topic or purpose of an email. It serves as the first impression and entices the recipient to open and read the email. A well-crafted email subject can effectively convey the urgency, relevance, and tone of the message, making it a crucial element in email communication. It is often the deciding factor in whether an email will be opened or ignored, making it a key component in successful communication and effective email marketing.