The best way to understand notion is as a relational database. You can build arbitrary collections (tables) of "cards" and choose structured properties (columns) on them. The "body" of the card itself is a standard markdown note.

The way you can display these cards is also very flexible (views): kanban, a table, list, gallery, etc with easy to setup filtering and sorting options. You can have multiple views attached to each collection (e.g. a KanBan board for team project managment and a separate "my TODOs" list)

The most powerful part is when you start to setup relations (foreign keys) between tables, which allows you to use all the power of relational data using GUI point and click. Anyone familiar with SQL (or Django ORM) will feel right at home with Foreign key, and reverse relationships.

The reason people are so excited about notion is that it "democratizes" the power of database — to non-technical people.