As far as I know, the crown of a rhubarb plant is the clump of its roots (when you go to buy rhubarb plants at the nursery, they're usually labeled as rhubarb "crowns").
Each clump of roots usually has one or more eyes (the eyes are buds, just like in potatoes).
If you are already growing rhubarb and you're wanting to divide it, you can just do a Google search and find several sites with instructions.
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