Beside the definite articles there are - as also in English - indefinite articles. In English there is (as also applies to the definite article) only one indefinite article a(n). Spanish has two because there is one needed for the feminine nouns and on for the masculine nouns.
Therefore there is una as indefinite article for feminine nouns and un for masculine ones.
Things get clearer when we play a mind game: A man who crosses the street is a friend of my mother. This sentence means that all men that cross the street are friends of my mother. And even if they had known my mother for 20 years they would not be friends unless they cross the street. In comparison: The man who crosses the street is a friend of my mother. This sentence seems quite clear. There is a man (one certain man) crossing the street, and exactly this man is befriend with my mother.
Actually there are no differences in the logic of using the indefinite articles between the Spanish and English. The only challenge here is to remember that for feminine nouns (the ones with the ending -a) get una. The others get un.

el vaso = the glass   un vaso = a glass

la vaca = the cow    una vaca = a cow

There are also plural forms of indefinite articles unas and unos. They are translated as some and also used the same way the little word some is used in English when meaning an indefinite quantity of something that can be counted. For substances that can not be counted (sand, milk or cream there is another word to be used in Spanish for meaning some. But we will get to this).

las paredes = the walls  unas paredes = some walls

los cuchillos = the knives  unos cuchillos = some knives

Let's have an overview on all the articles! Even though on first glance it seems a bit strange it's not that difficult as you will see:

la feminine la casa la luna la alegría house moon pleasure el* masculine el bolígrafo el paisaje el destino pen landscape aim, destiny

las feminine las casas las lunas las alegrías houses moons pleasures los masculine los bolígrafos los paisajes los regalos pens landscapes gifts

una feminine una casa una luna una alegría a house a moon a pleasure un masculine un bolígrafo un paisaje un destino a pen a landscape a faith

unas   feminine unas casas unas lunas unas alegrías some houses some moons some pleasures unos   masculine unos bolígrafos unos paisajes unos regalos some pens some landscapes some gifts

él = he   él trabaja = he works el hombre = (the) man