In English, the indefinite article a becomes an when it precedes a vowel. In Portuguese, the indefinite article must always agree with both the gender and number (singular or plural) of the noun. While the indefinite article has no plural in English, in Portuguese, plural forms of um and uma (uns and umas) are used to express indefinite plurals.

um avião = the airplane (m; sing.)
uns aviões = the airplanes (m; pl.)

um barco = the boat (m; sing.)
uns barcos = the boats (m; pl.)

uma fada = the fairy (f; sing)
umas fadas = the fairies (f; pl.)

uma bolsa = the purse (f; sing.)
umas bolsas = the purses (f; pl.)