Learn French: Ceux qui poussent sous le pin sont souvent comestibles. - The ones that grow under the pine are often edible.

We use qui because it acts as the subject of the verb poussent (they are the ones doing the growing). If they were the object of the verb—for example, the ones that I pick—you would use que instead (ceux que je ramasse).

What exactly does ceux replace here, and could I use celles?

Ceux means 'the ones' and replaces a masculine plural noun that was already mentioned in the conversation, such as les champignons (mushrooms). If you were talking about a feminine plural noun, like les plantes (plants), you would need to use celles instead.

Doesn't poussent normally mean 'push'? Why is it translated as 'grow' here?

Yes, the verb pousser often means 'to push', but it is also the standard verb used for plants, hair, and teeth 'growing' (conceptually, they are pushing out of the ground or skin). If you are talking about people or animals growing up, you must use the verb grandir instead.

Why is there an -s at the end of comestibles?

Because it is an adjective describing ceux, which is a plural pronoun. In French, adjectives must agree in number and gender with the noun or pronoun they describe, so we add an -s to make it plural.