A table tag within email content is how you can utilize the data within a selected relational table from your account, rather than using contact tags which are based on custom fields in your Maropost account. Table tags are specific to the table that you plan on utilizing within your content.
The following is a screenshot of WYSIWYG content editor with Table Tags drop-down highlighted:

Maropost for Marketing uses the Liquid script to display data from relational tables within the email body.
For all table tags, the middle section is the table’s name, and the end section is the field that you want to be displayed in the body of the email. The following is an example of table tag: {{table.table_name.registered_course}}. Here, table_name is the table’s name and registered_course is the field within the relational table that you wish to utilize.