How to use definite articles “o, os, a, as”
Definite articles are the equivalent to the English word "the", however, in Portuguese it can vary depending on gender and number. Every single object, idea or person is either masculine or feminine, except in very particular cases. The definite article helps you then to determine if the noun is feminine (a) or masculine (o). Plural feminine (as) and plural masculine (os).
Example: o travesseiro/the pillow (masculine singular) and os travesseiros/the pillows (masculine plural) a cama/the bed (feminine singular) and as camas/the beds (feminine plural)
Just as a complement, to make easier for portuguese non-native speakers, you just has to look at the word. Did you used the word "the" in the english sentece? Ok. Now look at the word after the "the". Is it in the plural? If yes, you gotta use "os" or "as". If not, you'll use "o" or "a". Now look at the word gender. Masculine? Use "o" or "os". Feminine? Use "a" or "as". A bit complicated? No.