It means your hips don't extend properly. Flexion is the forward movement - bending to touch your toes, or bringing your knee up like marching. Extension is teh opposite - bring your foot backwards behind your bottom.
If you have a fixed flexion deformity then try to stand straight and bring your leg behind you - hold something for balance support - and see if you can do this without leaning forward. You will be unable to if you have the deformity. - there are other ways to test this but just get someone to look at you while you do that and tell you.
it's often seen post hip replacement or in the elderly. generally it's a result of tight anterior structures or flexors of the hip - tight iliopsoas for example.
If you want to fix it then stretching is the way to go (depending on the exact cause for your deformity)... depending on your ability you could lie on your stomach with a pillow or 2 under your chest so your back is slightly arched, let your hips sink into the floor/bed, this will stretch the hips.
You can also try lying flat on your stomache and actively attempting to raise your leg off the bed - this strengthens the gluteals and and would help with any muscle inbalance.
In standing you can stretch one at a time - but you need to learn to do a posterior pelvic tilt (tucking your tail bone under)... let us know if you want more info