A foreign key is a Data Base concept used to ensure [referential integrity].  A foreign key means that values in one table must also appear in another table.

The referenced table is called the parent table while the table with the foreign key is called the child table. The foreign key in the child table will generally reference a primary key in the parent table.

Each table in the application schema has a field called [ID] which uniquely identifies each record.  Any linked tables will refer to this record, through this unique value using a foreign key.

for example,

The [P2K_CM_UNITS] table has a field [ID] which uniquely identifies a record.
The [P2K_CM_JOBS] table is linked to the units table through a foreign key link called [DUN_ID].  This link allows us to determine the unit that a job may be related to, by following the foreign key link.  It also ensures that a job is not "orphaned" by having the unit record deleted.

Each foreign key link is also identified with an index and constraint name which allows us to determine the relationship that is in question.  The [table alias|TABLE_ALIAS] that is on each side of this parent/child relationship is used in the naming of the foreign key constraint.

In the example above the foreign key constraint name would be defined from DJB (the alias for the Jobs table - the child table) and DUN (the alias for the units table - the parent table) thusly:  __P2K_DJB_DUN_FK1__.