Learn French: Ceux dont le pied est gros ne sont pas toujours comestibles. - The ones whose stem is big are not always edible.

Dont is the relative pronoun used for possession, meaning "whose" or "of which". So, ceux dont translates to "the ones whose". If we used qui, it would mean "the ones who" or "the ones that" acting as a subject, and que would be used if "the ones" were the object of the next verb.

Ceux is plural, so why does the sentence use the singular verb est?

The verb est belongs to the noun inside the relative clause: le pied (the stem), which is singular. The plural verb that matches Ceux (the ones) comes later in the sentence: ne sont pas (are not).

Why is gros singular while comestibles is plural?

The adjectives agree with different things in the sentence. Gros (big) describes le pied (the stem), which is singular. Comestibles (edible) describes the main subject Ceux (the ones), which is plural. Remember that gros naturally ends in an "s" even in its masculine singular form!

I thought le pied meant "the foot". Why is it translated as "stem" here?

Yes, le pied usually means "the foot", but it is also the standard French word for the "stem" or "stalk" of a mushroom, plant, or even a wine glass. Because this sentence talks about things that are comestibles (edible), we can tell it is referring to mushrooms!