'en' is an indefinite article used for masculin and feminin words in swedish (grammatical gender rather than real gender). It translates as "a"-"an" in english.
'ett' is also indefinite article amd will translate also as "a"-"an" but is used for neutral words (ett barn(child) for instance). There is no equivalent in french or english but if you are familiar with dutch or german, they have also neutral words such as huis in dutch (house). Difference is in german and dutch, is that you can't really see it with the undetermined article but with the determined article (such as "das" in german or "het" in dutch).
In Swedish and English, we have a gender neutral word barn / 'child', which translate each other. In Spanish for instance, they use the word niño for this purpose although it also means 'boy'. So if you're translating between Spanish and English, niño translates as either child or boy, and it's the same if you translate from Spanish into Swedish, niño means both barn and pojke. But if you translate between English and Swedish, barn only means 'child' and pojke is only 'boy'.