'The' is the definite article is English. It is used to restrict the meaning of a noun to make it refer to something that is known by both the speaker or writer and the listener or reader:
He's gone to the shops. (Here the listener knows which shops I mean)
It can also be used to refer back to something that has already been mentioned:
There's a word for that. Now, what is the word?
It can be used to refer forwards to something that is coming:
The key to the front door is under the mat.
It can be used to refer to a group:
The car has changed our way of living.