An indefinite article is a small tool word that always comes before a noun. It indicates the gender and number of the noun. Whereas in English there is only one definite article (a and its variation an), in French, there are three (un, une, des), depending on if the word is masculine, feminine or plural.
Un chat dort. (A cat is sleeping)
*Le chat is a masculine noun.
Une souris se cache. (A mouse is hiding)
*La souris is a feminine noun.
Des chiens aboient. (some dogs are barking)
*Les chiens is a masculine noun… but in this case, it doesn't matter ! What matters is that there are many dogs.
• We use a definite article when the noun refers to someone or something not clearly identified, which we haven't talked about yet.
Il y a un lapin. (There is a rabbit)
*we are not talking about a specific rabbit.
Le nom d'une ville. (The name of a city)
*we are not talking about one specific city.
• Be careful ! When a masculin singular noun begins with a vowel or a silent h, we pronounce the final letter of the article. We call this a liaison.
For singular masculine nouns , we pronounce the final letter of un : [n].
Un‿arbre Un‿hôpitalUn‿animalFor plural nouns , we pronounce the final letter of des : [z].
Des‿amisDes‿enfants*Don't forget it : when a noun is plural, it takes a silent -s marker at the end. Forgetting this poor silent -s is one of the most common mistakes among french speakers (natives or not!).