"She is eating."
The verb endings change depending on who does something (I, you, he/she/it ...). In addition, "essen" is an irregular ("strong") verb. Like some other irregular verbs, it changes its vowel for the "du" and "er/sie/es" forms in the present tense. Here, the vowel change is from "e" to "i", so it's "du isst" and not "du esst", etc. Unfortunately, the vowel changes of irregular verbs have to be learnt by heart.
essen (to eat)
Actually, according to the new spelling rules, it isn't, or rather, it's dated :). After short vowels, "ss" is now used instead of "ß": daß- dass, ißt-isst, Kuß - Kuss, etc. But maybe Duolingo still accepts the old spelling.
If anybody is wondering why we're having this discussion: the spelling reform is a fairly recent development (1996-2006).
The English progressive aspect (e.g. she is eating, he is reading, I am cooking) can't be translated literally into German. In fact, Standard German doesn't distinguish between the simple and the progressive aspects - there is just one form for both. So, depending on the context, "Sie isst" can be translated as "She eats" or as "She is eating/She's eating".
(In addition, in some regions in Germany there is also a special progressive form that is used in colloquial speech. I think Duolingo accepts it, but I would advise against using it. This colloquial progressive form, too, is not a literal translation from English: She is eating = colloquial progressive: Sie ist am Essen.)