"It is an apple."
Translation:C'est une pomme.
In English as well as in French, there are 2 main types of articles: indefinite and definite.
indefinite = an apple / une pomme, a tree / un arbre... meaning "one". ->note that in the plural form, there is no indefinite article in English, but in French, the plural of un/une is "des"
definite = the apple / la pomme, the tree / l'arbre... meaning one in particular, which is "defined" by context: the apple that is on the table (not an apple among others but a special one). -> note that in the plural form, "the" remains "the", but le/la become "les" -> also note that le or la in front of a word starting with a vowel or a non aspired H, become l' (L + apostrophe)