Tu is used for singular 'you' in the familiar form Io mangio, tu mangi, lei mangia, noi mangiamo, voi mangiate (is the plural you, you all, loro mangiano. I eat, you (familiar), you/he/she/it eats, (singular), we eat etc.
Was I the only one who misread the chicken as children and was really confused that there was no ragazzi in the answer building blocks because it seemed perfectly plausible that duolingo would ask me to translate that?
It was said in the basic lessons, in order to prevent any confusion betwen singular and plural 'you', that for 'voi' we will use 'you all' when we translate. So this was clearly a mistake on Duolingo's part. Both 'tu mangi" and "voi mangiate' should have been accepted.