Those are the two indefinite articles in Swedish, that is, like a and an in English.
Getting them right is often really difficult for someone trying to learn Swedish. They represent grammatical gender, which is not the same as biological gender, and which we don’t have in English. Grammatical gender, I mean.
Think of Spanish…the words that are grammatically masculine, like burro, are el burro. The words that are grammatically feminine, like cucaracha, are la cucaracha. For the handful of gender neutral words there’s lo, as in lo mismo.
Swedish has all three, masculine, feminine, and gender neutral, but unusually the masculine and feminine have in most cases combined, if you’ll excuse the expression. So a word is either with gender or without. If it has gender, it gets “en” as the indefinite article, if it doesn’t have gender, then it’s “ett”.
And, rather handy, for the definite article, the, you basically just put the article at the end. So a man in Swedish is “en man” and the man is “mannen”.
There are still some traces of masculine and feminine though. For example, the word for person, människa, is feminine.
Plurals are interesting in Swedish. In English, where you usually stick an “s” or maybe an “es” on the end of a word, in Swedish it’s “r” preceded by a vowel, which can be “a” or “e” or sometimes “o”. Getting those right can also be a real challenge for a learner.
And just as some plurals in English are the same as the singular, like deer or sheep, Swedish has that too, but it’s more or less all the “ett” words and only “ett” words. Barn, the word for children is both singular and plural, and so too is the word for sheep, får.
Actually there are a handful of words that can be either “ett” and “en”, but let’s not go there.
And finally, the gender neutral form, “ett” is also the number one in Swedish.