Email open rate refers to the percentage of recipients who open an email out of the total number of emails delivered. It is a key metric used to measure the success of an email marketing campaign and indicates the level of engagement and interest from the audience. A high email open rate suggests that the email subject line and content were compelling enough to capture the attention of the recipients, while a low open rate may indicate that the email was not relevant or interesting to the audience. Email open rate is an important factor in determining the effectiveness of email marketing efforts and can help businesses improve their email strategies to better connect with their target audience.