The U-value is the inverse of the R-Value; i.e. you divide 1 by either the R or U value to convert to the other unit. So as the R-Value goes up the U-Value goes down and as the R-Value goes down the U-Value goes up. So the U-Value is a measure of how well a material transmits heat.

For instance a substance with an R-Value of 2 has a U-Value of 0.5 = (1 divided by 2).