Absolute measures of dispersion focus on one set of observations, while relative focuses on two or more at a time.
Absolute measures use the same units as in the observations, but relative measures are free to use any unit for calculation.
Absolute measures are used when trying to find standard deviation and variance, so you may see it in your math class when you discuss probability.
Relative measures are used for the COEFFICIENTS of standard deviation/variance, which is defined as a ratio between the standard deviation and the mean. And not just standard deviation, I believe there's a bunch of other things too, I'm just not sure what since I haven't studied probability in depth.