The indefinite articles are un (masculine singular), une (feminine singular), and des (masculine, feminine plural). The indefinite article agrees in number and (for the singular forms) in gender with the nouns they modify (see Nouns):
The indefinite article is used to refer to objects, people, and events which have not been clearly defined by the speaker or the context. The nouns which they modify are generally countable nouns (unlike those modified by the partitive article). The indefinite article is frequently used after verbs expressing possession or consumption.
Donnez-moi un café, s'il vous plaît. (Give me a coffee, please.)
On peut faire d'une pierre deux coups. (One can hit two birds with one stone.)
Elle a des cerises à vendre. (She has cherries for sale.)