Swedish nouns have genders, by that I mean, articles and adjectives will slightly change depending on what gender the word is.
There are only two genders, "Ett" nouns (Neuter), and "En" nouns (Common).
So "Ett grönt äpple" -> "A green apple", but "En grön man" -> "A green man".
As for how do you know when a word is an "En" noun rather than an "Ett" noun?
There aren't really any concrete rules. It's mostly just memorization.