Why vote this guy down for asking a question? Especially when the rules haven't been explained yet. At this stage in the development of the Polish course on duolingo, apart from the first section, all that is possible is learning by example, like a child does naturally when he/she learns a language. If the rules are not being explained, people are going to have questions like this. I know I do.
This is way more complicated, generally speaking, adjectives that have R before the Y ending will change R to RZ for the masculine personal plural. Polish has 2 types of plural in the nominative case, one is the regular plural, which happens to be the same as the neutrer singular, so dobre jedzenie, dobre kanapki. But in case the noun is masculine personal, like ludzie, the last consonant becomes soft and according to some rules, the may Y changes into I. Just another example of Polish being super friendly to newcomers :)
https://en.m.wikibooks.org/w/index.php?title=Polish/Adjectives#Adjectives "Some softening rules..." at the end of the page, useful reference :)