What are fact table and Dimension table in Tableau?

These are data-warehouse terms rather than Tableau features, but Tableau is designed around them.

A fact table holds the events you measure — order lines, transactions, support tickets. It is long and narrow: mostly foreign keys plus numeric values.

A dimension table holds the things you slice by — Product, Customer, Date, Store. It is short and wide: one row per entity, with many descriptive attributes.

Joined one-to-many from dimension to fact, they form a star schema, which is the shape Tableau performs best against. Facts generally become measures in the Data pane and dimension attributes become dimensions, though Tableau assigns those roles by data type rather than by table.