pood
Explanation
The Joke
The comic presents two real facts: the "pood" is a unit of measurement equal to approximately 16 kilograms, and the "batman" is a unit of measurement equal to approximately 1 kilogram. It then draws the logical conclusion that for any equation containing a mass-squared term, you can express your answers in "batman-poods." A teacher is shown at a chalkboard with Newton's law of gravitation (F = G m1 m2 / r^2) saying, "Please express your answers with batman-poods."
Since Newton's gravitational formula multiplies two masses together (m1 times m2), the resulting unit for the mass-squared portion would be kilograms squared. But if you substitute one mass in batmans and one in poods, the product gives you "batman-poods" -- a unit that is both technically valid and sounds completely absurd.
The Humor
The humor is pure nerd comedy, exploiting the fact that obscure but real units of measurement happen to have names that sound ridiculous in combination. "Batman-poods" sounds like a nonsense phrase a child would make up, but it is a dimensionally correct unit for mass-squared (approximately 16 kg^2). The teacher's deadpan insistence on using this unit in a physics class elevates the absurdity. SMBC frequently delights in finding real but obscure facts and pushing them to their logical, silly conclusions.
References
The pood (Russian: pud) is a traditional Russian unit of mass equal to about 16.38 kilograms, used historically in Russia and still referenced in kettlebell training. The batman (or batsman) is an obsolete Ottoman/Persian unit of mass that varied by region but was roughly 1 kilogram in some definitions. Newton's law of universal gravitation, F = G(m1*m2)/r^2, describes the gravitational force between two masses.