Learn Italian: Le pesche sono buone: sono di stagione. - The peaches are good: they are in season.

Pesche is the plural of la pesca (the peach). In Italian, feminine words ending in -ca change to -che in the plural to keep the hard "k" sound. If you forgot the "h", you would write pesce, which means "fish"!

Why is the adjective buone instead of buoni?

Adjectives always have to match the gender and number of the noun they describe. Because le pesche is feminine and plural, the adjective buono (good) changes its ending to the feminine plural form buone.

Why do we use di instead of in for "in season"?

Prepositions rarely translate perfectly word-for-word. In Italian, the fixed expression for produce being "in season" is di stagione (literally "of season"). It is best to memorize this as a set phrase.

Where is the word for "they" in the second half of the sentence?

Italian drops subject pronouns like loro (they) most of the time. The verb sono means "they are" (or "I am"), and the context of talking about peaches makes it perfectly clear that it means "they are" here.