Dynamic content in email marketing includes custom HTML that changes based on each user. In most cases, it is used for personalization, which is one of the biggest factors of conversion improvement in email marketing. Dynamic content can be used in all aspects of the mail including the subject line and the body text.
A prime example of this is when marketers use your full name/first name to address the mail. This can be done simply by using a merge tag in your email service provider.
However, the scope of dynamic content that can be used in emails is rapidly expanding, and with the help of emerging technologies, we might be able to figure out how to implement more dynamism into marketing emails. Some of the recent developments have also allowed marketers to use localized subject lines and different CTA copies for different demographics.