"The wolf is eating a reindeer."
Translation:Ulven spiser en rein.
It's a (less common) alternative to "spiser".
People usually use one or the other, depending on their dialect, and it's not advisable to mix the two. Exceptions are made for the compound nouns and adjectives that strictly use "-eter" rather than "-spiser"; kjøtteter, planteeter, etc.
"et" is the indefinite article for neuter nouns.
"en" is the indefinite article for masculine nouns, and may also be used for feminine nouns.
"ei" is the indefinite article for feminine nouns.
All nouns have a grammatical gender which needs to be learned by heart, so it's a good idea to memorise each noun with its correspending article: "en gutt", "et hus", and so on.