- Articles are used to help us understand specific vs. general information. (=use 'the' for specific and 'a/an' for general information)

We use 'the' when we are thinking of a specific thing and when it is clear in the situation which thing or person we mean.

For example:

'Can you turn off the light, please?' (=the light in this room)

'I took a taxi to the station.' (=the station in that town)

'Tom sat down on the chair nearest the door' (=a specific chair)

'Tom sat down on a chair.' (=perhaps one of many chairs in the room)

'I'd like to speak to the manager, please' (=the manager of this shop/company, etc.)

- Articles also help us differentiate between new and old information. 

'I had a sandwich and an apple for lunch.' (=it is the first time the speaker talks about them) The sandwich wasn't very tasty, but the apple was fine' (=the listener knows which sandwich and which apple the speaker means, and this is old information)

I think of a/an as like saying there's a cloud of X's and we're going to pick out out at random, where as "the" is more like an arrow pointing to a particular X.

I think the official definition is with "the" you know which X you are talking about, and with "a/an" you don't.

It was clear to understand.