In English the indefinite articles are 'a', 'an'. In Portuguese language the indefinite articles are 'um', 'uma', 'uns', 'umas'.
'um' is used for indicating an indefinite object which is masculine. (um gato - a cat)
If the object is feminine, 'uma' is used like (uma caneta - a pen).
If masculine plural objects are used, it should be 'uns' like (uns gatos - some cats).
For feminine plural it is 'umas' like (umas canetas - some pens).