The two main uses of the definite article, 'the', are (1) with nouns which have been mentioned previously in the sentence/conversation:
'I saw
Terminator 3 last night.
The movie was very exciting.'
-- and (2) with nouns which are specific:
'I saw
Terminator 3 last night.
The lead actor was very big. (There is usually only one lead actor in a movie.)
Here is some more information on
THE DEFINITE ARTICLE