My orders said I am entitled to one- half separation pay what does that mean and how they came by that?

The quick answer is you must be ineligible to serve three years in the reserves. The long answer is look in AR 637-2

Half Pay - To qualify for half pay, the member must be involuntarily separated, with service characterized as Honorable or General (under honorable conditions), and the reason for discharge must be under certain categories. Examples would be discharge due to failing fitness/weight standards or involuntary discharge due to parenthood.