Placing Rhubarb in the crafting grid gives you a Rhubarb Seed.
Rhubarb can be planted directly onto or turned into seeds to plant onto tilled soil.
Once they're fully grown you can right-click them, harvest the Rhubarb and let them grow again. When harvested by right-click, the plant doesn't break, and yields 1-3 Rhubarb stalks.
Combining Rhubarb in a crafting grid with other Ingredients creates: