Complex sentence: Relative low income refers to people living in households with income below 60% of the median in a specific year whereas absolute low income refers to people living in households with income below 60% of median income in some base year.
Split into simpler sentences: Relative low income means that people live in households with income below 60% of the median in a specified year. Absolute income means that people live in households with income below 60% of the median income in some base year.