If a slider is in absolute mode, it's value will always be exactly where your finger touches it. It will react instantly. This is a good way to control parameters that you plan on making quick, large changes to. The drawback is that it's very sensitive and not very good for small, precise adjustments.
A slider in relative mode works differently. It moves the value relative to what your finger is doing, but there isn't a 1 to 1 correlation like there is in absolute mode. So if a slider is at 0 and you run your finger all the way to the top, it won't change the slider to it's maximum value. Instead, it will move the slider's value in relation to the direction and amount your finger moved. So it might move from 0 to 64. Relative mode is good for parameters that you know you won't be making large quick changes to, and is more suitable for precise control.