Tim, the zero is called a "subscript" in math. You will notice that it is written slightly below the line. In this case it represents the density of air in standard conditions at sea level. So it is a reference value and when the ratio of (r/r0) is written it is the ratio of the air density (r) to the reference air density (r0). [You wrote it as r0/r but I'm thinking it should be r/r0, but I could be wrong.] The term r0 is pronounced "r sub zero."
Usually, in engineering texts, the Greek letter rho (looks kinda like a "p") is used to express air density rather than the letter "r."
You didn't ask about the 1/2 but in the equation you reference, the 1/2 is an exponent of the ratio within the parentheses. An exponent of 1/2 is another way to say "the square root of."
Tim, I hate to dampen your enthusiasm, but when you fly your RV-12, you will not think about EAS. You may run across it on an FAA exam, but there are more challenging things to learn when you become a pilot.
If you can play a violin, I believe you can fly an RV-12.
