How can I make the relationship between rows in two tables more obvious to the user?

1. Which key creates relationship between two tables?
2. What is the significance of relationship between the tables?

Which key creates relationship between two tables?

Composite Keys and Lookup Columns

To create a relationship between two tables that have multiple columns defining the primary and foreign keys, first combine the values to create a single key column before creating the relationship.

What is the significance of relationship between the tables?

Relationships between tables tell you how much of the data from a foreign key field can be seen in the related primary key column and vice versa.