Articles are very important in English. They give some information about a person, thing or place. They stand in front of a noun or an adjective.
When we talk about something in general, we use the indefinite article (a / an), but when we talk about something specific, we use the definite article (the):
Please, bring me a pencil. (= any pencil)
Please, bring me the yellow pencil that is on my desk. (= not just any pencil, but the one that is yellow and on my desk)
To get a better view on English articles, watch these videos: