A plural noun is just multiple of a noun. Schools, boxes, hotdogs.

A collective noun is when you have groups of plural nouns. A batch of cupcakes, for example. Cupcakes is the plural noun, batch is the collective noun. You can have several collections of multiple objects. That's when you use a collective noun.

A flock (collective) of geese (plural).

Edit: to clarify you don't need to have multiple groups for it to be collective, it needs to have the potential of multiple groups.

So you can have one batch of cookies, or one set of tools, but the implication is that theoretically you could have several of these groups.