An indefinite article is used when referring to a general noun rather than a particular noun. While definite articles are used with specific nouns that are understood by both speaker and listener (the being the only English definite article), indefinite articles are used to call upon unspecified people or things.
In English, the main indefinite articles are a and an, while a few other words such as some and any can also fill the role. In French, the indefinite articles are un, une, and des.
un and une
Un is used before singular masculine nouns, and une is used before singular feminine nouns—for example:
J’ai un livre.
(I have a book.)
Je veux acheter une maison.
(I want to buy a house.)
In these sentences, neither livre nor maison applies to a particular item that is understood by the listener. Livre is a singular masculine noun, so it is preceded by un; maison is a singular feminine noun, so it is preceded by une.
As an indefinite article, des is the equivalent of the English some or any. It’s used before plural nouns of either gender—for example:
Nous avons acheté des livres.
(We bought books.)
For the most part, French speakers use an article before every verb. Whereas in English we might say,
We bought books, magazines, a pen, and a chair,
in French we would say,
Nous avons acheté des livres, des magazines, un stylo, et une chaise.