"It is not good to sniff food"
Translation:Si vizuri kunusa chakula
The vi- form is used to create adverbs from a lot of adjectives. Vizuri can be the equivalent of "well", as in Wewe huimba vizuri (You sing well)
As for why it's in this sentence, I believe it's because the sentence essentially doesn't have a subject (ie. there's no word equivalent to "it"), so the adverb form is used as the adjective has no noun or pronoun to agree with.
You could also put the infinitive/gerund phrase first, in which case -zuri will agree with it:
Kunusa chakula si kuzuri. (Sniffing food is not good.)