Line regulation is the ability of the power supply to maintain its specified output voltage over changes in the input line voltage. It is expressed as percent of change in the output voltage relative to the change in the input line voltage.
Load regulation is the ability of the power supply to maintain its specified output voltage given changes in the load. This does not mean the tolerance applies when there are sudden changes in load, it means over the permissible load range the regulation can change by this amount.
N.B. Both values apply to the output! This means the worst case value you need to use is both of the line and load tolerance percentages added together.
Take an unregulated 12V output DCDC specified with:
Line regulation: 1.2%
Load regulation: 7.5%
So for changes in line input voltage the output voltage can vary by +-144mV and for changes in line load output voltage can vary by +-900mV. So the actual output voltage will be in the range of 10.97V to 13.04V