Article introduces a noun and indicates how specific the noun is.
English has two articles: the definite article "the" and the indefinite article "a" (or "an").

INDEFINITE ARTICLE (A, AN)

Use a (or an) only before singular count nouns.

A (or an) means, approximately, "one." "I saw a cat" and "i saw one cat" describe the same event, but "I saw one cat" emphasizes that there was only one cat, not two.

Use the indefinite article to introduce...

1. Nonspecific, unknown nouns:

• He entered a brown building. 
• There is an apple on the tree.

2. One of a general group:

• A computer can do many things.