Rhubarb is the stem of the rhubarb plant and it's a bit ambiguous> Rhubarb is sometimes considered a fruit though we eat the stem, it's also sometimes considered a vegetable. Rhubarb is too sour to eat on its own. Rhubarb is boiled or stewed, usually with sugar but sweetners can also be used. Rhubarb can also be baked in pies, crumbles.
Fresh rhubarb is available roughly from late January/early February to June in the northern hemisphere. Forced rhubarb comes earlier in the season, has yellow leaves and a gentler flavour. Maincrop rhubarb comes later, is tougher and has a stronger flavour.  Rhubarb can be frozen for use out of season.  You can also buy frozen rhubarb if you're lucky at a shop near you. If there is no shop selling frozen rhubarb near you you can still buy frozen rhubarb online.