I'm going to repeat here what romapancake said so it's more visible:
"Le raisin" refers to a bunch of grapes as a single entity. Just one grape would be "un grain de raisin."
Not exactly, I'm pretty sure it refers to one bunch. I'm pretty sure it's like this:
le raisin = 1 bunch of grapes (as in all attached to each other)
les raisins = multiple bunches of grapes
le grain de raisin = 1 grape
les grains de raisin = multiple grapes, not connected to each other (maybe in a bowl or something)
I think the easiest way to wrap your head around that idea is that, in French, raisin is treated as a collective noun (like rice), but in English, a grape is a single berry. If you wanted to talk about a single grain of rice in English, you'd have to call it that : a grain of rice, not a rice.
So, in French, you have :
- Du raisin : grapes, in the general sense (unspecified quantity and presentation)
- Une grappe de raisin : a bunch/cluster or grapes
- Un grain/une baie de raisin : a single grape berry
- Le raisin : the grapes (general term, could be all the grapes in the universe, defines the concept of grapes, not one berry, as in "I like grapes", not as in "I eat one grape")
- Raisin sec : raisin, unintuitively, this one is not a collective noun, so if we say un raisin sec, it really represents a single raisin.