For growth traits, the units are pounds. This means that he is expected to produce calves 1.5 pounds heavier on average than the theoretical bull with a zero EPD for birth weight. As you can see, EPD's are designed to compare animals, nothing more. EPDs are calculated using statistical procedures that fall into a general category call Best Linear Unbiased Prediction or BLUP for short. BLUP procedures properly weigh vast amounts of information on individuals and their relatives.
Ratios are measures of individual performance and are calculated on a within-herd basis.
They contain no pedigree or progeny information and are really only appropriate for comparing animals within the same herd or contemporary group. EPDs are comparable across herds. The procedures used to calculate EPDs account for the merit of the mates to which an animal is bred. BLUP procedures account for environmental differences through information on the genetic relationships between animals in different herds. The more closely related the animals in two herds are, the better job BLUP procedures can do in adjusting for the differences in environment between those herds.