What is it and what number range would be considered to be high?

Copied from http://adamone.rchomepage.com/index3.htm:

"Aspect Ratio is a measure of how long and slender a wing is from tip to tip.
The Aspect Ratio of a wing is defined to be the square of the span divided by the wing area and is given the symbol AR.
The formula is simplified for a rectangular wing, as being the ratio of the span to the chord length."

Sport models: AR = 5 ~ 7

Sailplanes: AR = 15 ~ 20