Absolute and Relative Accuracy of DAC
You measure what each output value of the DAC is and record it, (also the supply voltage to help remove that from causing the deviations)

Absolute error is how large a deviation was there between what the value it output should be and what was measured, e.g. code 256 should have been 5V, you measure 4.96V, you run through your list of values, find the largest one and that is your absolute error

Relative error is how much does that deviation change between codes, e.g. code 256 - 255 should be X voltage apart, what was the largest deviation between 2 codes, that is your relative error.