What does it mean when you have knee pain when you straighten it & put weight on it?

Varies. Multiple potential causes including some degree of osteoarthritis in the knee or even a degenerative tear of the meniscus. Don't hesitate to be seen for s specific diagnosis.
Let try to help. When you straighten the knee you move your joint, to me that means some problem in the bone or the cartilage or both. To find the answer should go to family Doctor and have check it out.