A value that can be used as the primary key into another table. Since the primary key can be used to look up the information in another table, this information does not need to be included - only the primary key.
For example, a person sending an e-mail need not include the entire text of a book in the e-mail. Instead, they can include the ISBN number of the book, and interested persons can then use the number to get information about the book - or even the book itself. The ISBN is the primary-key of the book, and it is used as a foreign-key in the e-mail.
Note that using a foreign key assumes its existence as a primary key somewhere else. Improper foreign key/primary key relationships are the source of many database problems.