The email response curve refers to the graphical representation of the relationship between the number of emails sent and the corresponding response rate. It illustrates the pattern of how recipients engage with emails over time, showing the peak periods of high response and the lulls of low response. This curve is a valuable tool for email marketers to analyze and optimize their email campaigns, as it helps them understand the effectiveness of their messaging and timing in eliciting a response from their audience. By studying the email response curve, businesses can make informed decisions on when and how to send emails to maximize engagement and achieve their desired outcomes.