Why does my knee feel like its going to give out have pain when I walk. Had to ACE wrap it?

Unstable knee. Degeneration in the knee, excessive fluid (effusion), injury or laxity to the ligament/tendons, weakness of muscles, excessive pain, even lumbar disc disease can cause knee instability and thus sensation or actual "giving-out" wrapping of the knee reduces the "loose-feeling" thus helps you feel more confident in your knee as you walk. I suggest consulting doc for an evaluation. Good luck.