A Barn is a unit of area used in particle physics, specifically particle accelerators like the LHC. It’s roughly the cross-section of a Uranium atom.
A Barn is pretty damn small.
A parsec is a unit of length used in astronomy, it’s roughly 3.26 light years, most of the way to the nearest star other than the Sun. A megaparsec is a million parsecs, which is nearly twice the distance to the Andromeda galaxy (at the time of writing…it’s heading towards us…)
A parsec is pretty damn big.
So what happens when you create a volume by multiplying them? A disc a barn across, extended into a cylinder a megaparsec long? You get a pretty useful measuring spoon, about 3ml.