Two databases are related when an entry, or multiple entries, in one relates to an entry in another. Use relation properties to connect databases in Notion.
For example, projects and tasks are two separate databases in Notion
Rollups in Notion allow you to view data from a related database.
For example, “task completion” on projects is a rollup displaying the aggregate status property of related tasks.
To create a relation property, go to the three-dot database options menu, click "Properties," and then "Add a new property". Select "Relation" as the property type, and choose the database you want to relate to.