Foregrounding and Backgrounding

Basic terms
subordinate
subordinate elements (phrase/clause)
main clause 

Subordinate elements
Subordinate elements are sometimes said to convey background information, i.e. the kind of information which is not central to the overall topic of the text.

Main clause
Main clauses, by contrast, are said to convey foreground information, i.e. information which is central to the overall topic.  


Explanation 

From the grammatical point of view a subordinate clause is subordinate in that it falls outside the main part of the sentence, and can only occur together with this main part.

From an informational point of view, a subordinate clause may be said to be informationally subordinate.

Consider the following example:

        In the early sixties, Ayatollah Khomeini led the movement against the Shah of Iran's 'White Revolution'.


Here, the time phrase 'In the early sixties' is subordinate, while the material carrying the main line of the story is in the main clause. This is typical in English.


Compare this sentence with the following:

        The early sixties was a period of leadership for Ayatollah Khomeini against the Shah of Iran's 'White Revolution'.


Here, the sentence is odd. While the ideas in the text tell us that it must be about Ayatollah Khomeini's life, the organization of these ideas suggests that the listed date ought to be the topic of the text.

The sentence upgrades the time element from the subordinate status to a main status discoursally.