They hold organized data and have integrations and views for those data. It's nothing fancy like a mature sql-database-system, but you have different high-level views you can configure through the interface. So you can make tables with filters, a kanban, calendar, timeline and some more[1]. People use it for task-lists, project-management, to manage their movie and book-lists, etc.
you can make tables, with headers and data.
so if youre a team you can make tasks, and then another one for team members
then you can combine the two, so basically assign people to tasks and easily filter through them using the notion gui
or you can do simple things like your own tasks, add books to a "books i've read" collection, or whatever.
https://www.notion.so/help/intro-to-databases
and of course you can then easily take data from the db, and put them into your notes as well, at least if i recall. i've only used notion a tiny bit