This term is mainly used to refer to when an email is personalized to a group of people.
Email personalization rules are defined to automatically tailor copy or creative of an email or newsletter based on the interests or profile of a user.
IF <job role> = “marketer” then display “copy or image relevant” to marketer (syntax varies by different platforms).
It’s like a mail-merge.
Another form of dynamic content insertion is Keyword insertion in Google Ads which lets you automatically update your ads with the keywords in your ad group that caused your ads to show. This can help make your ads more relevant to users searching for what you offer and so increase clickthrough rate.