Quadriceps refers to a group of muscles in the front of your thigh: vastus medialis, vastus intermedius, vastus lateralis and rectus femoris.
All muscles are the primary produces of knee extension (ie. straightening the knee) while the rectus femoris muscle also crosses the hip joint and is therefore responsible for flexing the hip (ie. lifting the knee) as well.
Poor length will affect your stride length and knee flexion. Poor strength will affect your ability to squat, bend, lift, ascend or descend stairs. Both poor length and strength can produce knee, hip or lower back pain.
How to stretch? Whilst standing, lift your foot behind you (bending your knee) and grab a hold of your foot. If you cannot reach, use a towel or place your foot on a chair or bench. Keep your knee pointing down and next to your other knee.
Feel free to try this stretch. You should feel the stretch in the front of the thigh. STOP IMMEDIATELY and do not continue if you experience ANY pain!