A relationship connects two tables without merging them. Drag a table onto the Data Source canvas next to the main table, outside the physical layer, and Tableau draws a line (people call it the noodle) and matches the tables on a field with the same name. Click the line to change the fields.
The tables stay separate. When a sheet uses fields from both, Tableau queries each table at the level of detail of the view and then combines the results. A region's target is counted once per region, however many order lines the region has. That's the main reason to prefer relationships to joins when tables have different levels of detail.
Related tables show up as their own folders in the Data pane. You use their fields like any other: a dimension like Manager labels the marks, a measure like Sales Target aggregates correctly at any level.
Filters still matter: a filter on Order Date limits orders, while a target that belongs to a whole region isn't split by month.