UEC, like any real-world money, is an abstract unit representing labor. Humans invented money so we can trade fractions of effort back and forth with a common, unified system.
In Star Citizen, "effort" is still largely undefined--especially as additional gameplay systems continue to come online. Our end-goal is to balance the amount of effort you put in for the results that you get out, and make a balanced economy.
This problem is incredibly complex. So complex that we made a simulation to help us identify what prices should be. Currently it has like, a million Quanta running around trying to figure out wages. It's called Quantum.