RELATIVE time means that you can set a Reminder 'X' minutes/hours/days/weeks to be 'fired' before the Task is due.
ABSOLUTE time means that you set the hour:min on the day that you want the reminder to be 'fired'.
As Randolph said, in other words - a relative reminder can only be set to a task that has a due time. For example, a task due "today" can't have a relative reminder because you can't set a reminder to "1 hour before" deadline if there is no deadline. If the task is due "today at 2pm", then such reminder can be set to 1 hour before and that would be "today at 1pm".
Absolute reminders don't require due dates or times for the task.