A relationship is one-to-many or many-to-one relationship between two entities. It can also be thought of as a data table foreign key relationship.
When a relationship is defined between two entities, the entity records with the one-to-many relationship will have related record grid links on the left of navigation pane of the record. This provides quick access to view records that are associated with the master record.
Furthermore, records with a many-to-one relation also have an attribute of type lookup available to be placed on their form. When placed on the form, this field provides a link to its parent record.
When creating a relationship, it is important to select a relevant 'Lookup Display Field' and 'Lookup Display Name' because this is the value that will be displayed on all lookup fields associated with that relationship.